cmd/doc: provide working directory to build.Import calls

The current cmd/doc implementation uses go/build.Import in a few
places to check whether a package is findable and importable.
go/build has limited support for finding packages in modules,
but to do so, build.Import requires knowing the source directory
to use when performing the lookup (so it can find the go.mod file).
Otherwise, it only looks inside the GOPATH workspace.

Start passing the current working directory to build.Import calls,
so that it can correctly look for packages in modules when in cmd/doc
is executed in module mode.

Before this change, cmd/doc in module mode could mistakenly find and
use a package in the GOPATH workspace, instead of the current module.

Since the result of os.Getwd is needed in even more places, assign it
to a local variable in parseArgs now.

Fixes #28992
Updates #26504

Change-Id: I7571618e18420d2d3b3890cc69ade2d97b1962bf
Reviewed-on: https://go-review.googlesource.com/c/go/+/183991
Run-TryBot: Dmitri Shuralyov <dmitshur@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>

src/cmd/doc/doc_test.go

@@ -920,7 +920,10 @@ func TestDotSlashLookup(t *testing.T) {
 		t.Skip("scanning file system takes too long")
 	}
 	maybeSkip(t)
-	where := pwd()
+	where, err := os.Getwd()
+	if err != nil {
+		t.Fatal(err)
+	}
 	defer func() {
 		if err := os.Chdir(where); err != nil {
 			t.Fatal(err)
@@ -931,7 +934,7 @@ func TestDotSlashLookup(t *testing.T) {
 	}
 	var b bytes.Buffer
 	var flagSet flag.FlagSet
-	err := do(&b, &flagSet, []string{"./template"})
+	err = do(&b, &flagSet, []string{"./template"})
 	if err != nil {
 		t.Errorf("unexpected error %q from ./template", err)
 	}

src/cmd/doc/main.go

@@ -187,16 +187,20 @@ func failMessage(paths []string, symbol, method string) error {
 // is rand.Float64, we must scan both crypto/rand and math/rand
 // to find the symbol, and the first call will return crypto/rand, true.
 func parseArgs(args []string) (pkg *build.Package, path, symbol string, more bool) {
+	wd, err := os.Getwd()
+	if err != nil {
+		log.Fatal(err)
+	}
 	if len(args) == 0 {
 		// Easy: current directory.
-		return importDir(pwd()), "", "", false
+		return importDir(wd), "", "", false
 	}
 	arg := args[0]
 	// We have an argument. If it is a directory name beginning with . or ..,
 	// use the absolute path name. This discriminates "./errors" from "errors"
 	// if the current directory contains a non-standard errors package.
 	if isDotSlash(arg) {
-		arg = filepath.Join(pwd(), arg)
+		arg = filepath.Join(wd, arg)
 	}
 	switch len(args) {
 	default:
@@ -205,7 +209,7 @@ func parseArgs(args []string) (pkg *build.Package, path, symbol string, more boo
 		// Done below.
 	case 2:
 		// Package must be findable and importable.
-		pkg, err := build.Import(args[0], "", build.ImportComment)
+		pkg, err := build.Import(args[0], wd, build.ImportComment)
 		if err == nil {
 			return pkg, args[0], args[1], false
 		}
@@ -225,7 +229,7 @@ func parseArgs(args []string) (pkg *build.Package, path, symbol string, more boo
 	// First, is it a complete package path as it is? If so, we are done.
 	// This avoids confusion over package paths that have other
 	// package paths as their prefix.
-	pkg, err := build.Import(arg, "", build.ImportComment)
+	pkg, err = build.Import(arg, wd, build.ImportComment)
 	if err == nil {
 		return pkg, arg, "", false
 	}
@@ -260,7 +264,7 @@ func parseArgs(args []string) (pkg *build.Package, path, symbol string, more boo
 			symbol = arg[period+1:]
 		}
 		// Have we identified a package already?
-		pkg, err := build.Import(arg[0:period], "", build.ImportComment)
+		pkg, err := build.Import(arg[0:period], wd, build.ImportComment)
 		if err == nil {
 			return pkg, arg[0:period], symbol, false
 		}
@@ -283,7 +287,7 @@ func parseArgs(args []string) (pkg *build.Package, path, symbol string, more boo
 		log.Fatalf("no such package %s", arg[0:period])
 	}
 	// Guess it's a symbol in the current directory.
-	return importDir(pwd()), "", arg, false
+	return importDir(wd), "", arg, false
 }
 
 // dotPaths lists all the dotted paths legal on Unix-like and
@@ -385,12 +389,3 @@ var buildCtx = build.Default
 func splitGopath() []string {
 	return filepath.SplitList(buildCtx.GOPATH)
 }
-
-// pwd returns the current directory.
-func pwd() string {
-	wd, err := os.Getwd()
-	if err != nil {
-		log.Fatal(err)
-	}
-	return wd
-}

src/cmd/go/testdata/script/mod_doc.txt

@@ -3,6 +3,7 @@
 env GO111MODULE=on
 [short] skip
 
+# Check when module x is inside GOPATH/src.
 go doc y
 stdout 'Package y is.*alphabet'
 stdout 'import "x/y"'
@@ -16,13 +17,25 @@ stdout 'Hello returns a greeting'
 go doc quote
 stdout 'Package quote collects pithy sayings.'
 
-# Double-check go doc y when y is not in GOPATH/src.
-env GOPATH=$WORK/altgopath
+# Double-check when module x is outside GOPATH/src.
+env GOPATH=$WORK/emptygopath
 go doc x/y
 stdout 'Package y is.*alphabet'
 go doc y
 stdout 'Package y is.*alphabet'
 
+# Triple-check when module x is outside GOPATH/src,
+# but other packages with same import paths are in GOPATH/src.
+# Since go doc is running in module mode here, packages in active module
+# should be preferred over packages in GOPATH. See golang.org/issue/28992.
+env GOPATH=$WORK/gopath2
+go doc x/y
+! stdout 'Package y is.*GOPATH'
+stdout 'Package y is.*alphabet'
+go doc rsc.io/quote
+! stdout 'Package quote is located in a GOPATH workspace.'
+stdout 'Package quote collects pithy sayings.'
+
 -- go.mod --
 module x
 require rsc.io/quote v1.5.2
@@ -33,3 +46,13 @@ package y
 
 -- x.go --
 package x
+
+-- $WORK/gopath2/src/x/y/y.go --
+// Package y is located in a GOPATH workspace.
+package y
+-- $WORK/gopath2/src/rsc.io/quote/quote.go --
+// Package quote is located in a GOPATH workspace.
+package quote
+
+// Hello is located in a GOPATH workspace.
+func Hello() string { return "" }