import.go

     1  // Copyright 2018 The Go Authors. All rights reserved.
     2  // Use of this source code is governed by a BSD-style
     3  // license that can be found in the LICENSE file.
     4  
     5  package modload
     6  
     7  import (
     8  	"context"
     9  	"errors"
    10  	"fmt"
    11  	"go/build"
    12  	"io/fs"
    13  	"os"
    14  	pathpkg "path"
    15  	"path/filepath"
    16  	"sort"
    17  	"strings"
    18  
    19  	"cmd/go/internal/cfg"
    20  	"cmd/go/internal/fsys"
    21  	"cmd/go/internal/gover"
    22  	"cmd/go/internal/modfetch"
    23  	"cmd/go/internal/modindex"
    24  	"cmd/go/internal/search"
    25  	"cmd/go/internal/str"
    26  	"cmd/internal/par"
    27  
    28  	"golang.org/x/mod/module"
    29  )
    30  
    31  type ImportMissingError struct {
    32  	Path     string
    33  	Module   module.Version
    34  	QueryErr error
    35  
    36  	ImportingMainModule module.Version
    37  
    38  	// isStd indicates whether we would expect to find the package in the standard
    39  	// library. This is normally true for all dotless import paths, but replace
    40  	// directives can cause us to treat the replaced paths as also being in
    41  	// modules.
    42  	isStd bool
    43  
    44  	// importerGoVersion is the version the module containing the import error
    45  	// specified. It is only set when isStd is true.
    46  	importerGoVersion string
    47  
    48  	// replaced the highest replaced version of the module where the replacement
    49  	// contains the package. replaced is only set if the replacement is unused.
    50  	replaced module.Version
    51  
    52  	// newMissingVersion is set to a newer version of Module if one is present
    53  	// in the build list. When set, we can't automatically upgrade.
    54  	newMissingVersion string
    55  }
    56  
    57  func (e *ImportMissingError) Error() string {
    58  	if e.Module.Path == "" {
    59  		if e.isStd {
    60  			msg := fmt.Sprintf("package %s is not in std (%s)", e.Path, filepath.Join(cfg.GOROOT, "src", e.Path))
    61  			if e.importerGoVersion != "" {
    62  				msg += fmt.Sprintf("\nnote: imported by a module that requires go %s", e.importerGoVersion)
    63  			}
    64  			return msg
    65  		}
    66  		if e.QueryErr != nil && e.QueryErr != ErrNoModRoot {
    67  			return fmt.Sprintf("cannot find module providing package %s: %v", e.Path, e.QueryErr)
    68  		}
    69  		if cfg.BuildMod == "mod" || (cfg.BuildMod == "readonly" && allowMissingModuleImports) {
    70  			return "cannot find module providing package " + e.Path
    71  		}
    72  
    73  		if e.replaced.Path != "" {
    74  			suggestArg := e.replaced.Path
    75  			if !module.IsZeroPseudoVersion(e.replaced.Version) {
    76  				suggestArg = e.replaced.String()
    77  			}
    78  			return fmt.Sprintf("module %s provides package %s and is replaced but not required; to add it:\n\tgo get %s", e.replaced.Path, e.Path, suggestArg)
    79  		}
    80  
    81  		message := fmt.Sprintf("no required module provides package %s", e.Path)
    82  		if e.QueryErr != nil {
    83  			return fmt.Sprintf("%s: %v", message, e.QueryErr)
    84  		}
    85  		if e.ImportingMainModule.Path != "" && e.ImportingMainModule != MainModules.ModContainingCWD() {
    86  			return fmt.Sprintf("%s; to add it:\n\tcd %s\n\tgo get %s", message, MainModules.ModRoot(e.ImportingMainModule), e.Path)
    87  		}
    88  		return fmt.Sprintf("%s; to add it:\n\tgo get %s", message, e.Path)
    89  	}
    90  
    91  	if e.newMissingVersion != "" {
    92  		return fmt.Sprintf("package %s provided by %s at latest version %s but not at required version %s", e.Path, e.Module.Path, e.Module.Version, e.newMissingVersion)
    93  	}
    94  
    95  	return fmt.Sprintf("missing module for import: %s@%s provides %s", e.Module.Path, e.Module.Version, e.Path)
    96  }
    97  
    98  func (e *ImportMissingError) Unwrap() error {
    99  	return e.QueryErr
   100  }
   101  
   102  func (e *ImportMissingError) ImportPath() string {
   103  	return e.Path
   104  }
   105  
   106  // An AmbiguousImportError indicates an import of a package found in multiple
   107  // modules in the build list, or found in both the main module and its vendor
   108  // directory.
   109  type AmbiguousImportError struct {
   110  	importPath string
   111  	Dirs       []string
   112  	Modules    []module.Version // Either empty or 1:1 with Dirs.
   113  }
   114  
   115  func (e *AmbiguousImportError) ImportPath() string {
   116  	return e.importPath
   117  }
   118  
   119  func (e *AmbiguousImportError) Error() string {
   120  	locType := "modules"
   121  	if len(e.Modules) == 0 {
   122  		locType = "directories"
   123  	}
   124  
   125  	var buf strings.Builder
   126  	fmt.Fprintf(&buf, "ambiguous import: found package %s in multiple %s:", e.importPath, locType)
   127  
   128  	for i, dir := range e.Dirs {
   129  		buf.WriteString("\n\t")
   130  		if i < len(e.Modules) {
   131  			m := e.Modules[i]
   132  			buf.WriteString(m.Path)
   133  			if m.Version != "" {
   134  				fmt.Fprintf(&buf, " %s", m.Version)
   135  			}
   136  			fmt.Fprintf(&buf, " (%s)", dir)
   137  		} else {
   138  			buf.WriteString(dir)
   139  		}
   140  	}
   141  
   142  	return buf.String()
   143  }
   144  
   145  // A DirectImportFromImplicitDependencyError indicates a package directly
   146  // imported by a package or test in the main module that is satisfied by a
   147  // dependency that is not explicit in the main module's go.mod file.
   148  type DirectImportFromImplicitDependencyError struct {
   149  	ImporterPath string
   150  	ImportedPath string
   151  	Module       module.Version
   152  }
   153  
   154  func (e *DirectImportFromImplicitDependencyError) Error() string {
   155  	return fmt.Sprintf("package %s imports %s from implicitly required module; to add missing requirements, run:\n\tgo get %s@%s", e.ImporterPath, e.ImportedPath, e.Module.Path, e.Module.Version)
   156  }
   157  
   158  func (e *DirectImportFromImplicitDependencyError) ImportPath() string {
   159  	return e.ImporterPath
   160  }
   161  
   162  // ImportMissingSumError is reported in readonly mode when we need to check
   163  // if a module contains a package, but we don't have a sum for its .zip file.
   164  // We might need sums for multiple modules to verify the package is unique.
   165  //
   166  // TODO(#43653): consolidate multiple errors of this type into a single error
   167  // that suggests a 'go get' command for root packages that transitively import
   168  // packages from modules with missing sums. load.CheckPackageErrors would be
   169  // a good place to consolidate errors, but we'll need to attach the import
   170  // stack here.
   171  type ImportMissingSumError struct {
   172  	importPath                string
   173  	found                     bool
   174  	mods                      []module.Version
   175  	importer, importerVersion string // optional, but used for additional context
   176  	importerIsTest            bool
   177  }
   178  
   179  func (e *ImportMissingSumError) Error() string {
   180  	var importParen string
   181  	if e.importer != "" {
   182  		importParen = fmt.Sprintf(" (imported by %s)", e.importer)
   183  	}
   184  	var message string
   185  	if e.found {
   186  		message = fmt.Sprintf("missing go.sum entry needed to verify package %s%s is provided by exactly one module", e.importPath, importParen)
   187  	} else {
   188  		message = fmt.Sprintf("missing go.sum entry for module providing package %s%s", e.importPath, importParen)
   189  	}
   190  	var hint string
   191  	if e.importer == "" {
   192  		// Importing package is unknown, or the missing package was named on the
   193  		// command line. Recommend 'go mod download' for the modules that could
   194  		// provide the package, since that shouldn't change go.mod.
   195  		if len(e.mods) > 0 {
   196  			args := make([]string, len(e.mods))
   197  			for i, mod := range e.mods {
   198  				args[i] = mod.Path
   199  			}
   200  			hint = fmt.Sprintf("; to add:\n\tgo mod download %s", strings.Join(args, " "))
   201  		}
   202  	} else {
   203  		// Importing package is known (common case). Recommend 'go get' on the
   204  		// current version of the importing package.
   205  		tFlag := ""
   206  		if e.importerIsTest {
   207  			tFlag = " -t"
   208  		}
   209  		version := ""
   210  		if e.importerVersion != "" {
   211  			version = "@" + e.importerVersion
   212  		}
   213  		hint = fmt.Sprintf("; to add:\n\tgo get%s %s%s", tFlag, e.importer, version)
   214  	}
   215  	return message + hint
   216  }
   217  
   218  func (e *ImportMissingSumError) ImportPath() string {
   219  	return e.importPath
   220  }
   221  
   222  type invalidImportError struct {
   223  	importPath string
   224  	err        error
   225  }
   226  
   227  func (e *invalidImportError) ImportPath() string {
   228  	return e.importPath
   229  }
   230  
   231  func (e *invalidImportError) Error() string {
   232  	return e.err.Error()
   233  }
   234  
   235  func (e *invalidImportError) Unwrap() error {
   236  	return e.err
   237  }
   238  
   239  // importFromModules finds the module and directory in the dependency graph of
   240  // rs containing the package with the given import path. If mg is nil,
   241  // importFromModules attempts to locate the module using only the main module
   242  // and the roots of rs before it loads the full graph.
   243  //
   244  // The answer must be unique: importFromModules returns an error if multiple
   245  // modules are observed to provide the same package.
   246  //
   247  // importFromModules can return a module with an empty m.Path, for packages in
   248  // the standard library.
   249  //
   250  // importFromModules can return an empty directory string, for fake packages
   251  // like "C" and "unsafe".
   252  //
   253  // If the package is not present in any module selected from the requirement
   254  // graph, importFromModules returns an *ImportMissingError.
   255  //
   256  // If the package is present in exactly one module, importFromModules will
   257  // return the module, its root directory, and a list of other modules that
   258  // lexically could have provided the package but did not.
   259  //
   260  // If skipModFile is true, the go.mod file for the package is not loaded. This
   261  // allows 'go mod tidy' to preserve a minor checksum-preservation bug
   262  // (https://go.dev/issue/56222) for modules with 'go' versions between 1.17 and
   263  // 1.20, preventing unnecessary go.sum churn and network access in those
   264  // modules.
   265  func importFromModules(ctx context.Context, path string, rs *Requirements, mg *ModuleGraph, skipModFile bool) (m module.Version, modroot, dir string, altMods []module.Version, err error) {
   266  	invalidf := func(format string, args ...interface{}) (module.Version, string, string, []module.Version, error) {
   267  		return module.Version{}, "", "", nil, &invalidImportError{
   268  			importPath: path,
   269  			err:        fmt.Errorf(format, args...),
   270  		}
   271  	}
   272  
   273  	if strings.Contains(path, "@") {
   274  		return invalidf("import path %q should not have @version", path)
   275  	}
   276  	if build.IsLocalImport(path) {
   277  		return invalidf("%q is relative, but relative import paths are not supported in module mode", path)
   278  	}
   279  	if filepath.IsAbs(path) {
   280  		return invalidf("%q is not a package path; see 'go help packages'", path)
   281  	}
   282  	if search.IsMetaPackage(path) {
   283  		return invalidf("%q is not an importable package; see 'go help packages'", path)
   284  	}
   285  
   286  	if path == "C" {
   287  		// There's no directory for import "C".
   288  		return module.Version{}, "", "", nil, nil
   289  	}
   290  	// Before any further lookup, check that the path is valid.
   291  	if err := module.CheckImportPath(path); err != nil {
   292  		return module.Version{}, "", "", nil, &invalidImportError{importPath: path, err: err}
   293  	}
   294  
   295  	// Check each module on the build list.
   296  	var dirs, roots []string
   297  	var mods []module.Version
   298  
   299  	// Is the package in the standard library?
   300  	pathIsStd := search.IsStandardImportPath(path)
   301  	if pathIsStd && modindex.IsStandardPackage(cfg.GOROOT, cfg.BuildContext.Compiler, path) {
   302  		for _, mainModule := range MainModules.Versions() {
   303  			if MainModules.InGorootSrc(mainModule) {
   304  				if dir, ok, err := dirInModule(path, MainModules.PathPrefix(mainModule), MainModules.ModRoot(mainModule), true); err != nil {
   305  					return module.Version{}, MainModules.ModRoot(mainModule), dir, nil, err
   306  				} else if ok {
   307  					return mainModule, MainModules.ModRoot(mainModule), dir, nil, nil
   308  				}
   309  			}
   310  		}
   311  		dir := filepath.Join(cfg.GOROOTsrc, path)
   312  		modroot = cfg.GOROOTsrc
   313  		if str.HasPathPrefix(path, "cmd") {
   314  			modroot = filepath.Join(cfg.GOROOTsrc, "cmd")
   315  		}
   316  		dirs = append(dirs, dir)
   317  		roots = append(roots, modroot)
   318  		mods = append(mods, module.Version{})
   319  	}
   320  	// -mod=vendor is special.
   321  	// Everything must be in the main modules or the main module's or workspace's vendor directory.
   322  	if cfg.BuildMod == "vendor" {
   323  		var mainErr error
   324  		for _, mainModule := range MainModules.Versions() {
   325  			modRoot := MainModules.ModRoot(mainModule)
   326  			if modRoot != "" {
   327  				dir, mainOK, err := dirInModule(path, MainModules.PathPrefix(mainModule), modRoot, true)
   328  				if mainErr == nil {
   329  					mainErr = err
   330  				}
   331  				if mainOK {
   332  					mods = append(mods, mainModule)
   333  					dirs = append(dirs, dir)
   334  					roots = append(roots, modRoot)
   335  				}
   336  			}
   337  		}
   338  
   339  		if HasModRoot() {
   340  			vendorDir := VendorDir()
   341  			dir, inVendorDir, _ := dirInModule(path, "", vendorDir, false)
   342  			if inVendorDir {
   343  				readVendorList(vendorDir)
   344  				// If vendorPkgModule does not contain an entry for path then it's probably either because
   345  				// vendor/modules.txt does not exist or the user manually added directories to the vendor directory.
   346  				// Go 1.23 and later require vendored packages to be present in modules.txt to be imported.
   347  				_, ok := vendorPkgModule[path]
   348  				if ok || (gover.Compare(MainModules.GoVersion(), gover.ExplicitModulesTxtImportVersion) < 0) {
   349  					mods = append(mods, vendorPkgModule[path])
   350  					dirs = append(dirs, dir)
   351  					roots = append(roots, vendorDir)
   352  				} else {
   353  					subCommand := "mod"
   354  					if inWorkspaceMode() {
   355  						subCommand = "work"
   356  					}
   357  					fmt.Fprintf(os.Stderr, "go: ignoring package %s which exists in the vendor directory but is missing from vendor/modules.txt. To sync the vendor directory run go %s vendor.\n", path, subCommand)
   358  				}
   359  			}
   360  		}
   361  
   362  		if len(dirs) > 1 {
   363  			return module.Version{}, "", "", nil, &AmbiguousImportError{importPath: path, Dirs: dirs}
   364  		}
   365  
   366  		if mainErr != nil {
   367  			return module.Version{}, "", "", nil, mainErr
   368  		}
   369  
   370  		if len(mods) == 0 {
   371  			return module.Version{}, "", "", nil, &ImportMissingError{Path: path}
   372  		}
   373  
   374  		return mods[0], roots[0], dirs[0], nil, nil
   375  	}
   376  
   377  	// Iterate over possible modules for the path, not all selected modules.
   378  	// Iterating over selected modules would make the overall loading time
   379  	// O(M × P) for M modules providing P imported packages, whereas iterating
   380  	// over path prefixes is only O(P × k) with maximum path depth k. For
   381  	// large projects both M and P may be very large (note that M ≤ P), but k
   382  	// will tend to remain smallish (if for no other reason than filesystem
   383  	// path limitations).
   384  	//
   385  	// We perform this iteration either one or two times. If mg is initially nil,
   386  	// then we first attempt to load the package using only the main module and
   387  	// its root requirements. If that does not identify the package, or if mg is
   388  	// already non-nil, then we attempt to load the package using the full
   389  	// requirements in mg.
   390  	for {
   391  		var sumErrMods, altMods []module.Version
   392  		for prefix := path; prefix != "."; prefix = pathpkg.Dir(prefix) {
   393  			if gover.IsToolchain(prefix) {
   394  				// Do not use the synthetic "go" module for "go/ast".
   395  				continue
   396  			}
   397  			var (
   398  				v  string
   399  				ok bool
   400  			)
   401  			if mg == nil {
   402  				v, ok = rs.rootSelected(prefix)
   403  			} else {
   404  				v, ok = mg.Selected(prefix), true
   405  			}
   406  			if !ok || v == "none" {
   407  				continue
   408  			}
   409  			m := module.Version{Path: prefix, Version: v}
   410  
   411  			root, isLocal, err := fetch(ctx, m)
   412  			if err != nil {
   413  				if sumErr := (*sumMissingError)(nil); errors.As(err, &sumErr) {
   414  					// We are missing a sum needed to fetch a module in the build list.
   415  					// We can't verify that the package is unique, and we may not find
   416  					// the package at all. Keep checking other modules to decide which
   417  					// error to report. Multiple sums may be missing if we need to look in
   418  					// multiple nested modules to resolve the import; we'll report them all.
   419  					sumErrMods = append(sumErrMods, m)
   420  					continue
   421  				}
   422  				// Report fetch error.
   423  				// Note that we don't know for sure this module is necessary,
   424  				// but it certainly _could_ provide the package, and even if we
   425  				// continue the loop and find the package in some other module,
   426  				// we need to look at this module to make sure the import is
   427  				// not ambiguous.
   428  				return module.Version{}, "", "", nil, err
   429  			}
   430  			if dir, ok, err := dirInModule(path, m.Path, root, isLocal); err != nil {
   431  				return module.Version{}, "", "", nil, err
   432  			} else if ok {
   433  				mods = append(mods, m)
   434  				roots = append(roots, root)
   435  				dirs = append(dirs, dir)
   436  			} else {
   437  				altMods = append(altMods, m)
   438  			}
   439  		}
   440  
   441  		if len(mods) > 1 {
   442  			// We produce the list of directories from longest to shortest candidate
   443  			// module path, but the AmbiguousImportError should report them from
   444  			// shortest to longest. Reverse them now.
   445  			for i := 0; i < len(mods)/2; i++ {
   446  				j := len(mods) - 1 - i
   447  				mods[i], mods[j] = mods[j], mods[i]
   448  				roots[i], roots[j] = roots[j], roots[i]
   449  				dirs[i], dirs[j] = dirs[j], dirs[i]
   450  			}
   451  			return module.Version{}, "", "", nil, &AmbiguousImportError{importPath: path, Dirs: dirs, Modules: mods}
   452  		}
   453  
   454  		if len(sumErrMods) > 0 {
   455  			for i := 0; i < len(sumErrMods)/2; i++ {
   456  				j := len(sumErrMods) - 1 - i
   457  				sumErrMods[i], sumErrMods[j] = sumErrMods[j], sumErrMods[i]
   458  			}
   459  			return module.Version{}, "", "", nil, &ImportMissingSumError{
   460  				importPath: path,
   461  				mods:       sumErrMods,
   462  				found:      len(mods) > 0,
   463  			}
   464  		}
   465  
   466  		if len(mods) == 1 {
   467  			// We've found the unique module containing the package.
   468  			// However, in order to actually compile it we need to know what
   469  			// Go language version to use, which requires its go.mod file.
   470  			//
   471  			// If the module graph is pruned and this is a test-only dependency
   472  			// of a package in "all", we didn't necessarily load that file
   473  			// when we read the module graph, so do it now to be sure.
   474  			if !skipModFile && cfg.BuildMod != "vendor" && mods[0].Path != "" && !MainModules.Contains(mods[0].Path) {
   475  				if _, err := goModSummary(mods[0]); err != nil {
   476  					return module.Version{}, "", "", nil, err
   477  				}
   478  			}
   479  			return mods[0], roots[0], dirs[0], altMods, nil
   480  		}
   481  
   482  		if mg != nil {
   483  			// We checked the full module graph and still didn't find the
   484  			// requested package.
   485  			var queryErr error
   486  			if !HasModRoot() {
   487  				queryErr = ErrNoModRoot
   488  			}
   489  			return module.Version{}, "", "", nil, &ImportMissingError{Path: path, QueryErr: queryErr, isStd: pathIsStd}
   490  		}
   491  
   492  		// So far we've checked the root dependencies.
   493  		// Load the full module graph and try again.
   494  		mg, err = rs.Graph(ctx)
   495  		if err != nil {
   496  			// We might be missing one or more transitive (implicit) dependencies from
   497  			// the module graph, so we can't return an ImportMissingError here — one
   498  			// of the missing modules might actually contain the package in question,
   499  			// in which case we shouldn't go looking for it in some new dependency.
   500  			return module.Version{}, "", "", nil, err
   501  		}
   502  	}
   503  }
   504  
   505  // queryImport attempts to locate a module that can be added to the current
   506  // build list to provide the package with the given import path.
   507  //
   508  // Unlike QueryPattern, queryImport prefers to add a replaced version of a
   509  // module *before* checking the proxies for a version to add.
   510  func queryImport(ctx context.Context, path string, rs *Requirements) (module.Version, error) {
   511  	// To avoid spurious remote fetches, try the latest replacement for each
   512  	// module (golang.org/issue/26241).
   513  	var mods []module.Version
   514  	if MainModules != nil { // TODO(#48912): Ensure MainModules exists at this point, and remove the check.
   515  		for mp, mv := range MainModules.HighestReplaced() {
   516  			if !maybeInModule(path, mp) {
   517  				continue
   518  			}
   519  			if mv == "" {
   520  				// The only replacement is a wildcard that doesn't specify a version, so
   521  				// synthesize a pseudo-version with an appropriate major version and a
   522  				// timestamp below any real timestamp. That way, if the main module is
   523  				// used from within some other module, the user will be able to upgrade
   524  				// the requirement to any real version they choose.
   525  				if _, pathMajor, ok := module.SplitPathVersion(mp); ok && len(pathMajor) > 0 {
   526  					mv = module.ZeroPseudoVersion(pathMajor[1:])
   527  				} else {
   528  					mv = module.ZeroPseudoVersion("v0")
   529  				}
   530  			}
   531  			mg, err := rs.Graph(ctx)
   532  			if err != nil {
   533  				return module.Version{}, err
   534  			}
   535  			if gover.ModCompare(mp, mg.Selected(mp), mv) >= 0 {
   536  				// We can't resolve the import by adding mp@mv to the module graph,
   537  				// because the selected version of mp is already at least mv.
   538  				continue
   539  			}
   540  			mods = append(mods, module.Version{Path: mp, Version: mv})
   541  		}
   542  	}
   543  
   544  	// Every module path in mods is a prefix of the import path.
   545  	// As in QueryPattern, prefer the longest prefix that satisfies the import.
   546  	sort.Slice(mods, func(i, j int) bool {
   547  		return len(mods[i].Path) > len(mods[j].Path)
   548  	})
   549  	for _, m := range mods {
   550  		root, isLocal, err := fetch(ctx, m)
   551  		if err != nil {
   552  			if sumErr := (*sumMissingError)(nil); errors.As(err, &sumErr) {
   553  				return module.Version{}, &ImportMissingSumError{importPath: path}
   554  			}
   555  			return module.Version{}, err
   556  		}
   557  		if _, ok, err := dirInModule(path, m.Path, root, isLocal); err != nil {
   558  			return m, err
   559  		} else if ok {
   560  			if cfg.BuildMod == "readonly" {
   561  				return module.Version{}, &ImportMissingError{Path: path, replaced: m}
   562  			}
   563  			return m, nil
   564  		}
   565  	}
   566  	if len(mods) > 0 && module.CheckPath(path) != nil {
   567  		// The package path is not valid to fetch remotely,
   568  		// so it can only exist in a replaced module,
   569  		// and we know from the above loop that it is not.
   570  		replacement := Replacement(mods[0])
   571  		return module.Version{}, &PackageNotInModuleError{
   572  			Mod:         mods[0],
   573  			Query:       "latest",
   574  			Pattern:     path,
   575  			Replacement: replacement,
   576  		}
   577  	}
   578  
   579  	if search.IsStandardImportPath(path) {
   580  		// This package isn't in the standard library, isn't in any module already
   581  		// in the build list, and isn't in any other module that the user has
   582  		// shimmed in via a "replace" directive.
   583  		// Moreover, the import path is reserved for the standard library, so
   584  		// QueryPattern cannot possibly find a module containing this package.
   585  		//
   586  		// Instead of trying QueryPattern, report an ImportMissingError immediately.
   587  		return module.Version{}, &ImportMissingError{Path: path, isStd: true}
   588  	}
   589  
   590  	if (cfg.BuildMod == "readonly" || cfg.BuildMod == "vendor") && !allowMissingModuleImports {
   591  		// In readonly mode, we can't write go.mod, so we shouldn't try to look up
   592  		// the module. If readonly mode was enabled explicitly, include that in
   593  		// the error message.
   594  		// In vendor mode, we cannot use the network or module cache, so we
   595  		// shouldn't try to look up the module
   596  		var queryErr error
   597  		if cfg.BuildModExplicit {
   598  			queryErr = fmt.Errorf("import lookup disabled by -mod=%s", cfg.BuildMod)
   599  		} else if cfg.BuildModReason != "" {
   600  			queryErr = fmt.Errorf("import lookup disabled by -mod=%s\n\t(%s)", cfg.BuildMod, cfg.BuildModReason)
   601  		}
   602  		return module.Version{}, &ImportMissingError{Path: path, QueryErr: queryErr}
   603  	}
   604  
   605  	// Look up module containing the package, for addition to the build list.
   606  	// Goal is to determine the module, download it to dir,
   607  	// and return m, dir, ImportMissingError.
   608  	fmt.Fprintf(os.Stderr, "go: finding module for package %s\n", path)
   609  
   610  	mg, err := rs.Graph(ctx)
   611  	if err != nil {
   612  		return module.Version{}, err
   613  	}
   614  
   615  	candidates, err := QueryPackages(ctx, path, "latest", mg.Selected, CheckAllowed)
   616  	if err != nil {
   617  		if errors.Is(err, fs.ErrNotExist) {
   618  			// Return "cannot find module providing package […]" instead of whatever
   619  			// low-level error QueryPattern produced.
   620  			return module.Version{}, &ImportMissingError{Path: path, QueryErr: err}
   621  		} else {
   622  			return module.Version{}, err
   623  		}
   624  	}
   625  
   626  	candidate0MissingVersion := ""
   627  	for i, c := range candidates {
   628  		if v := mg.Selected(c.Mod.Path); gover.ModCompare(c.Mod.Path, v, c.Mod.Version) > 0 {
   629  			// QueryPattern proposed that we add module c.Mod to provide the package,
   630  			// but we already depend on a newer version of that module (and that
   631  			// version doesn't have the package).
   632  			//
   633  			// This typically happens when a package is present at the "@latest"
   634  			// version (e.g., v1.0.0) of a module, but we have a newer version
   635  			// of the same module in the build list (e.g., v1.0.1-beta), and
   636  			// the package is not present there.
   637  			if i == 0 {
   638  				candidate0MissingVersion = v
   639  			}
   640  			continue
   641  		}
   642  		return c.Mod, nil
   643  	}
   644  	return module.Version{}, &ImportMissingError{
   645  		Path:              path,
   646  		Module:            candidates[0].Mod,
   647  		newMissingVersion: candidate0MissingVersion,
   648  	}
   649  }
   650  
   651  // maybeInModule reports whether, syntactically,
   652  // a package with the given import path could be supplied
   653  // by a module with the given module path (mpath).
   654  func maybeInModule(path, mpath string) bool {
   655  	return mpath == path ||
   656  		len(path) > len(mpath) && path[len(mpath)] == '/' && path[:len(mpath)] == mpath
   657  }
   658  
   659  var (
   660  	haveGoModCache   par.Cache[string, bool]    // dir → bool
   661  	haveGoFilesCache par.ErrCache[string, bool] // dir → haveGoFiles
   662  )
   663  
   664  // dirInModule locates the directory that would hold the package named by the given path,
   665  // if it were in the module with module path mpath and root mdir.
   666  // If path is syntactically not within mpath,
   667  // or if mdir is a local file tree (isLocal == true) and the directory
   668  // that would hold path is in a sub-module (covered by a go.mod below mdir),
   669  // dirInModule returns "", false, nil.
   670  //
   671  // Otherwise, dirInModule returns the name of the directory where
   672  // Go source files would be expected, along with a boolean indicating
   673  // whether there are in fact Go source files in that directory.
   674  // A non-nil error indicates that the existence of the directory and/or
   675  // source files could not be determined, for example due to a permission error.
   676  func dirInModule(path, mpath, mdir string, isLocal bool) (dir string, haveGoFiles bool, err error) {
   677  	// Determine where to expect the package.
   678  	if path == mpath {
   679  		dir = mdir
   680  	} else if mpath == "" { // vendor directory
   681  		dir = filepath.Join(mdir, path)
   682  	} else if len(path) > len(mpath) && path[len(mpath)] == '/' && path[:len(mpath)] == mpath {
   683  		dir = filepath.Join(mdir, path[len(mpath)+1:])
   684  	} else {
   685  		return "", false, nil
   686  	}
   687  
   688  	// Check that there aren't other modules in the way.
   689  	// This check is unnecessary inside the module cache
   690  	// and important to skip in the vendor directory,
   691  	// where all the module trees have been overlaid.
   692  	// So we only check local module trees
   693  	// (the main module, and any directory trees pointed at by replace directives).
   694  	if isLocal {
   695  		for d := dir; d != mdir && len(d) > len(mdir); {
   696  			haveGoMod := haveGoModCache.Do(d, func() bool {
   697  				fi, err := fsys.Stat(filepath.Join(d, "go.mod"))
   698  				return err == nil && !fi.IsDir()
   699  			})
   700  
   701  			if haveGoMod {
   702  				return "", false, nil
   703  			}
   704  			parent := filepath.Dir(d)
   705  			if parent == d {
   706  				// Break the loop, as otherwise we'd loop
   707  				// forever if d=="." and mdir=="".
   708  				break
   709  			}
   710  			d = parent
   711  		}
   712  	}
   713  
   714  	// Now committed to returning dir (not "").
   715  
   716  	// Are there Go source files in the directory?
   717  	// We don't care about build tags, not even "go:build ignore".
   718  	// We're just looking for a plausible directory.
   719  	haveGoFiles, err = haveGoFilesCache.Do(dir, func() (bool, error) {
   720  		// modindex.GetPackage will return ErrNotIndexed for any directories which
   721  		// are reached through a symlink, so that they will be handled by
   722  		// fsys.IsDirWithGoFiles below.
   723  		if ip, err := modindex.GetPackage(mdir, dir); err == nil {
   724  			return ip.IsDirWithGoFiles()
   725  		} else if !errors.Is(err, modindex.ErrNotIndexed) {
   726  			return false, err
   727  		}
   728  		return fsys.IsDirWithGoFiles(dir)
   729  	})
   730  
   731  	return dir, haveGoFiles, err
   732  }
   733  
   734  // fetch downloads the given module (or its replacement)
   735  // and returns its location.
   736  //
   737  // The isLocal return value reports whether the replacement,
   738  // if any, is local to the filesystem.
   739  func fetch(ctx context.Context, mod module.Version) (dir string, isLocal bool, err error) {
   740  	if modRoot := MainModules.ModRoot(mod); modRoot != "" {
   741  		return modRoot, true, nil
   742  	}
   743  	if r := Replacement(mod); r.Path != "" {
   744  		if r.Version == "" {
   745  			dir = r.Path
   746  			if !filepath.IsAbs(dir) {
   747  				dir = filepath.Join(replaceRelativeTo(), dir)
   748  			}
   749  			// Ensure that the replacement directory actually exists:
   750  			// dirInModule does not report errors for missing modules,
   751  			// so if we don't report the error now, later failures will be
   752  			// very mysterious.
   753  			if _, err := fsys.Stat(dir); err != nil {
   754  				// TODO(bcmills): We should also read dir/go.mod here and check its Go version,
   755  				// and return a gover.TooNewError if appropriate.
   756  
   757  				if os.IsNotExist(err) {
   758  					// Semantically the module version itself “exists” — we just don't
   759  					// have its source code. Remove the equivalence to os.ErrNotExist,
   760  					// and make the message more concise while we're at it.
   761  					err = fmt.Errorf("replacement directory %s does not exist", r.Path)
   762  				} else {
   763  					err = fmt.Errorf("replacement directory %s: %w", r.Path, err)
   764  				}
   765  				return dir, true, module.VersionError(mod, err)
   766  			}
   767  			return dir, true, nil
   768  		}
   769  		mod = r
   770  	}
   771  
   772  	if mustHaveSums() && !modfetch.HaveSum(mod) {
   773  		return "", false, module.VersionError(mod, &sumMissingError{})
   774  	}
   775  
   776  	dir, err = modfetch.Download(ctx, mod)
   777  	return dir, false, err
   778  }
   779  
   780  // mustHaveSums reports whether we require that all checksums
   781  // needed to load or build packages are already present in the go.sum file.
   782  func mustHaveSums() bool {
   783  	return HasModRoot() && cfg.BuildMod == "readonly" && !inWorkspaceMode()
   784  }
   785  
   786  type sumMissingError struct {
   787  	suggestion string
   788  }
   789  
   790  func (e *sumMissingError) Error() string {
   791  	return "missing go.sum entry" + e.suggestion
   792  }
   793
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