cmd/go: use index to match packages in dependency modules

If we're trying to search in a module in the module cache, instead
iterate over the packages in the index.
Change-Id: Ia94cbe6e9690110c28b93dbb33810680e3010381
Reviewed-on: https://go-review.googlesource.com/c/go/+/403756
Reviewed-by: Michael Matloob <matloob@golang.org>
Reviewed-by: Peter Weinberger <pjw@google.com>
Run-TryBot: Michael Matloob <matloob@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>

src/cmd/go/internal/modindex/read.go

@@ -210,7 +210,7 @@ func (mi *ModuleIndex) Packages() []string {
 
 // RelPath returns the path relative to the module's root.
 func (mi *ModuleIndex) RelPath(path string) string {
-	return filepath.Clean(str.TrimFilePathPrefix(path, mi.modroot))
+	return str.TrimFilePathPrefix(path, mi.modroot)
 }
 
 // ImportPackage is the equivalent of build.Import given the information in ModuleIndex.

src/cmd/go/internal/modindex/scan.go

@@ -4,6 +4,7 @@ import (
 	"cmd/go/internal/base"
 	"cmd/go/internal/fsys"
 	"cmd/go/internal/par"
+	"cmd/go/internal/str"
 	"encoding/json"
 	"errors"
 	"fmt"
@@ -54,10 +55,10 @@ func indexModule(modroot string) ([]byte, error) {
 		if !info.IsDir() {
 			return nil
 		}
-		rel, err := filepath.Rel(modroot, path)
-		if err != nil {
-			panic(err)
+		if !str.HasFilePathPrefix(path, modroot) {
+			panic(fmt.Errorf("path %v in walk doesn't have modroot %v as prefix:", path, modroot))
 		}
+		rel := str.TrimFilePathPrefix(path, modroot)
 		packages = append(packages, importRaw(modroot, rel))
 		return nil
 	})

src/cmd/go/internal/modload/search.go

@@ -6,15 +6,18 @@ package modload
 
 import (
 	"context"
+	"errors"
 	"fmt"
 	"io/fs"
 	"os"
+	"path"
 	"path/filepath"
 	"strings"
 
 	"cmd/go/internal/cfg"
 	"cmd/go/internal/fsys"
 	"cmd/go/internal/imports"
+	"cmd/go/internal/modindex"
 	"cmd/go/internal/search"
 
 	"golang.org/x/mod/module"
@@ -165,6 +168,12 @@ func matchPackages(ctx context.Context, m *search.Match, tags map[string]bool, f
 			}
 			modPrefix = mod.Path
 		}
+		if mi, err := modindex.Get(root); err == nil {
+			walkFromIndex(ctx, m, tags, root, mi, have, modPrefix)
+			continue
+		} else if !errors.Is(err, modindex.ErrNotIndexed) {
+			m.AddError(err)
+		}
 
 		prune := pruneVendor
 		if isLocal {
@@ -176,6 +185,60 @@ func matchPackages(ctx context.Context, m *search.Match, tags map[string]bool, f
 	return
 }
 
+// walkFromIndex matches packages in a module using the module index. modroot
+// is the module's root directory on disk, index is the ModuleIndex for the
+// module, and importPathRoot is the module's path prefix.
+func walkFromIndex(ctx context.Context, m *search.Match, tags map[string]bool, modroot string, index *modindex.ModuleIndex, have map[string]bool, importPathRoot string) {
+	isMatch := func(string) bool { return true }
+	treeCanMatch := func(string) bool { return true }
+	if !m.IsMeta() {
+		isMatch = search.MatchPattern(m.Pattern())
+		treeCanMatch = search.TreeCanMatchPattern(m.Pattern())
+	}
+loopPackages:
+	for _, reldir := range index.Packages() {
+		// Avoid .foo, _foo, and testdata subdirectory trees.
+		p := reldir
+		for {
+			elem, rest, found := strings.Cut(p, string(filepath.Separator))
+			if strings.HasPrefix(elem, ".") || strings.HasPrefix(elem, "_") || elem == "testdata" {
+				continue loopPackages
+			}
+			if found && elem == "vendor" {
+				// Ignore this path if it contains the element "vendor" anywhere
+				// except for the last element (packages named vendor are allowed
+				// for historical reasons). Note that found is true when this
+				// isn't the last path element.
+				continue loopPackages
+			}
+			if !found {
+				// Didn't find the separator, so we're considering the last element.
+				break
+			}
+			p = rest
+		}
+
+		// Don't use GOROOT/src.
+		if reldir == "" && importPathRoot == "" {
+			continue
+		}
+
+		name := path.Join(importPathRoot, filepath.ToSlash(reldir))
+		if !treeCanMatch(name) {
+			continue
+		}
+
+		if !have[name] {
+			have[name] = true
+			if isMatch(name) {
+				if _, _, err := index.ScanDir(reldir, tags); err != imports.ErrNoGo {
+					m.Pkgs = append(m.Pkgs, name)
+				}
+			}
+		}
+	}
+}
+
 // MatchInModule identifies the packages matching the given pattern within the
 // given module version, which does not need to be in the build list or module
 // requirement graph.