cmd/go: report scan error position in 'go list -e'tls

This CL extracts some error handling code into a common method for
presenting errors encountered when loading package data.

Fixes #36087
Fixes #36762

Change-Id: I87c8d41e3cc6e6afa152d9c067bc60923bf19fbe
Reviewed-on: https://go-review.googlesource.com/c/go/+/210938
Run-TryBot: Jay Conrod <jayconrod@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Michael Matloob <matloob@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>

4 files changed, 99 insertions, 61 deletions

diff --git a/src/cmd/go/internal/base/base.go b/src/cmd/go/internal/base/base.go
index 272da55681..ab2f1bb4e2 100644
--- a/src/cmd/go/internal/base/base.go
+++ b/src/cmd/go/internal/base/base.go
@@ -7,11 +7,8 @@
 package base
 
 import (
-	"bytes"
-	"errors"
 	"flag"
 	"fmt"
-	"go/scanner"
 	"log"
 	"os"
 	"os/exec"
@@ -172,25 +169,3 @@ func RunStdin(cmdline []string) {
 // Usage is the usage-reporting function, filled in by package main
 // but here for reference by other packages.
 var Usage func()
-
-// ExpandScanner expands a scanner.List error into all the errors in the list.
-// The default Error method only shows the first error
-// and does not shorten paths.
-func ExpandScanner(err error) error {
-	// Look for parser errors.
-	if err, ok := err.(scanner.ErrorList); ok {
-		// Prepare error with \n before each message.
-		// When printed in something like context: %v
-		// this will put the leading file positions each on
-		// its own line. It will also show all the errors
-		// instead of just the first, as err.Error does.
-		var buf bytes.Buffer
-		for _, e := range err {
-			e.Pos.Filename = ShortPath(e.Pos.Filename)
-			buf.WriteString("\n")
-			buf.WriteString(e.Error())
-		}
-		return errors.New(buf.String())
-	}
-	return err
-}
diff --git a/src/cmd/go/internal/load/pkg.go b/src/cmd/go/internal/load/pkg.go
index 6aea54340d..247f5ed506 100644
--- a/src/cmd/go/internal/load/pkg.go
+++ b/src/cmd/go/internal/load/pkg.go
@@ -210,21 +210,68 @@ func (e *NoGoError) Error() string {
 	return "no Go files in " + e.Package.Dir
 }
 
-// rewordError returns a version of err with trivial layers removed and
-// (possibly-wrapped) instances of build.NoGoError replaced with load.NoGoError,
-// which more clearly distinguishes sub-cases.
-func (p *Package) rewordError(err error) error {
-	if mErr, ok := err.(*search.MatchError); ok && mErr.Match.IsLiteral() {
-		err = mErr.Err
-	}
-	var noGo *build.NoGoError
-	if errors.As(err, &noGo) {
-		if p.Dir == "" && noGo.Dir != "" {
-			p.Dir = noGo.Dir
+// setLoadPackageDataError presents an error found when loading package data
+// as a *PackageError. It has special cases for some common errors to improve
+// messages shown to users and reduce redundancy.
+//
+// setLoadPackageDataError returns true if it's safe to load information about
+// imported packages, for example, if there was a parse error loading imports
+// in one file, but other files are okay.
+//
+// TODO(jayconrod): we should probably return nothing and always try to load
+// imported packages.
+func (p *Package) setLoadPackageDataError(err error, path string, stk *ImportStack) (canLoadImports bool) {
+	// Include the path on the import stack unless the error includes it already.
+	errHasPath := false
+	if impErr, ok := err.(ImportPathError); ok && impErr.ImportPath() == path {
+		errHasPath = true
+	} else if matchErr, ok := err.(*search.MatchError); ok && matchErr.Match.Pattern() == path {
+		errHasPath = true
+		if matchErr.Match.IsLiteral() {
+			// The error has a pattern has a pattern similar to the import path.
+			// It may be slightly different (./foo matching example.com/foo),
+			// but close enough to seem redundant.
+			// Unwrap the error so we don't show the pattern.
+			err = matchErr.Err
+		}
+	}
+	var errStk []string
+	if errHasPath {
+		errStk = stk.Copy()
+	} else {
+		stk.Push(path)
+		errStk = stk.Copy()
+		stk.Pop()
+	}
+
+	// Replace (possibly wrapped) *build.NoGoError with *load.NoGoError.
+	// The latter is more specific about the cause.
+	var nogoErr *build.NoGoError
+	if errors.As(err, &nogoErr) {
+		if p.Dir == "" && nogoErr.Dir != "" {
+			p.Dir = nogoErr.Dir
 		}
 		err = &NoGoError{Package: p}
 	}
-	return err
+
+	// Take only the first error from a scanner.ErrorList. PackageError only
+	// has room for one position, so we report the first error with a position
+	// instead of all of the errors without a position.
+	var pos string
+	if scanErr, ok := err.(scanner.ErrorList); ok && len(scanErr) > 0 {
+		scanPos := scanErr[0].Pos
+		scanPos.Filename = base.ShortPath(scanPos.Filename)
+		pos = scanPos.String()
+		err = errors.New(scanErr[0].Msg)
+		canLoadImports = true
+	}
+
+	p.Error = &PackageError{
+		ImportStack: errStk,
+		Pos:         pos,
+		Err:         err,
+	}
+	return canLoadImports
 }
 
 // Resolve returns the resolved version of imports,
@@ -1554,21 +1601,10 @@ func (p *Package) load(path string, stk *ImportStack, bp *build.Package, err err
 
 	if err != nil {
 		p.Incomplete = true
-		// Report path in error stack unless err is an ImportPathError with path already set.
-		pushed := false
-		if e, ok := err.(ImportPathError); !ok || e.ImportPath() != path {
-			stk.Push(path)
-			pushed = true // Remember to pop after setError.
-		}
-		setError(base.ExpandScanner(p.rewordError(err)))
-		if pushed {
-			stk.Pop()
-		}
-		if _, isScanErr := err.(scanner.ErrorList); !isScanErr {
+		canLoadImports := p.setLoadPackageDataError(err, path, stk)
+		if !canLoadImports {
 			return
 		}
-		// Fall through if there was an error parsing a file. 'go list -e' should
-		// still report imports and other metadata.
 	}
 
 	useBindir := p.Name == "main"
@@ -2136,10 +2172,8 @@ func PackagesAndErrors(patterns []string) []*Package {
 			// Report it as a synthetic package.
 			p := new(Package)
 			p.ImportPath = m.Pattern()
-			p.Error = &PackageError{
-				ImportStack: nil, // The error arose from a pattern, not an import.
-				Err:         p.rewordError(m.Errs[0]),
-			}
+			var stk ImportStack // empty stack, since the error arose from a pattern, not an import
+			p.setLoadPackageDataError(m.Errs[0], m.Pattern(), &stk)
 			p.Incomplete = true
 			p.Match = append(p.Match, m.Pattern())
 			p.Internal.CmdlinePkg = true
diff --git a/src/cmd/go/internal/load/test.go b/src/cmd/go/internal/load/test.go
index 6465f46f4e..1c0d01c16c 100644
--- a/src/cmd/go/internal/load/test.go
+++ b/src/cmd/go/internal/load/test.go
@@ -6,7 +6,6 @@ package load
 
 import (
 	"bytes"
-	"cmd/go/internal/base"
 	"cmd/go/internal/str"
 	"errors"
 	"fmt"
@@ -271,7 +270,9 @@ func TestPackagesAndErrors(p *Package, cover *TestCover) (pmain, ptest, pxtest *
 	// afterward that gathers t.Cover information.
 	t, err := loadTestFuncs(ptest)
 	if err != nil && pmain.Error == nil {
-		pmain.Error = &PackageError{Err: err}
+		_ = pmain.setLoadPackageDataError(err, p.ImportPath, &stk)
+		// Ignore return value. None of the errors from loadTestFuncs should prevent
+		// us from loading information about imports.
 	}
 	t.Cover = cover
 	if len(ptest.GoFiles)+len(ptest.CgoFiles) > 0 {
@@ -540,7 +541,7 @@ var testFileSet = token.NewFileSet()
 func (t *testFuncs) load(filename, pkg string, doImport, seen *bool) error {
 	f, err := parser.ParseFile(testFileSet, filename, nil, parser.ParseComments)
 	if err != nil {
-		return base.ExpandScanner(err)
+		return err
 	}
 	for _, d := range f.Decls {
 		n, ok := d.(*ast.FuncDecl)
diff --git a/src/cmd/go/testdata/script/list_parse_err.txt b/src/cmd/go/testdata/script/list_parse_err.txt
index 5aacaa88fa..3c5345801a 100644
--- a/src/cmd/go/testdata/script/list_parse_err.txt
+++ b/src/cmd/go/testdata/script/list_parse_err.txt
@@ -1,17 +1,45 @@
-# 'go list' should report imports, even if some files have parse errors
+# 'go list' without -e should fail and print errors on stderr.
+! go list ./p
+stderr '^p[/\\]b.go:2:2: expected ''package'', found ''EOF''$'
+! go list -f '{{range .Imports}}{{.}} {{end}}' ./p
+stderr '^p[/\\]b.go:2:2: expected ''package'', found ''EOF''$'
+! go list -test ./t
+stderr '^can''t load test package: t[/\\]t_test.go:8:1: expected declaration, found ʕ'
+! go list -test -f '{{range .Imports}}{{.}} {{end}}' ./t
+stderr '^can''t load test package: t[/\\]t_test.go:8:1: expected declaration, found ʕ'
+
+# 'go list -e' should report imports, even if some files have parse errors
 # before the import block.
-go list -e -f '{{range .Imports}}{{.}} {{end}}'
+go list -e -f '{{range .Imports}}{{.}} {{end}}' ./p
 stdout '^fmt '
 
+# 'go list' should report the position of the error if there's only one.
+go list -e -f '{{.Error.Pos}} => {{.Error.Err}}' ./p
+stdout 'b.go:[0-9:]+ => expected ''package'', found ''EOF'''
+
+# 'go test' should report the position of the error if there's only one.
+go list -e -test -f '{{if .Error}}{{.Error.Pos}} => {{.Error.Err}}{{end}}' ./t
+stdout 't_test.go:[0-9:]+ => expected declaration, found ʕ'
+
 -- go.mod --
 module m
 
 go 1.13
 
--- a.go --
+-- p/a.go --
 package a
 
 import "fmt"
 
--- b.go --
+-- p/b.go --
 // no package statement
+
+-- t/t_test.go --
+package t
+
+import "testing"
+
+func Test(t *testing.T) {}
+
+// scan error
+ʕ◔ϖ◔ʔ