cmd/go: make malformed go.sum a fatal error

In CL 197062, many errors related to go.sum were changed from base.Fatal
to error returns. The malformed go.sum error was lost in the process.
Currently, when go encounters a malformed go.sum file, go will read the
well-formed part of the file and then silently ignore the rest.

The motivation behind moving away from base.Fatal was to make the errors
show up in -json output. Simply propagating the malformed go.sum error
would not achieve this:

- For an argument-less 'go mod download -json' with a go>=1.17 module,
  a malformed go.sum causes an error during LoadModGraph already, before
  go ever starts downloading modules and printing their JSON.
- In other cases, a malformed go.sum would be reported as Error for one
  of the modules (presumably the one which gets downloaded first) but
  none of the others.
- In either case, 'go mod download' manages to download enough data to
  succeed on a re-run, making the error look intermittent.

Switch the error back to a Fatal one, but give 'go mod tidy' an
exception to let it fix broken go.sum files.

Fixes #62345

Author: chmeliik

src/cmd/go/internal/modfetch/fetch.go

@@ -504,7 +504,7 @@ const emptyGoModHash = "h1:G7mAYYxgmS0lVkHyy2hEOLQCFB0DlQFTMLWggykrydY="
 
 // readGoSum parses data, which is the content of file,
 // and adds it to goSum.m. The goSum lock must be held.
-func readGoSum(dst map[module.Version][]string, file string, data []byte) error {
+func readGoSum(dst map[module.Version][]string, file string, data []byte) {
 	lineno := 0
 	for len(data) > 0 {
 		var line []byte
@@ -521,7 +521,12 @@ func readGoSum(dst map[module.Version][]string, file string, data []byte) error
 			continue
 		}
 		if len(f) != 3 {
-			return fmt.Errorf("malformed go.sum:\n%s:%d: wrong number of fields %v", file, lineno, len(f))
+			if cfg.CmdName == "mod tidy" {
+				// ignore malformed line so that go mod tidy can fix go.sum
+				continue
+			} else {
+				base.Fatalf("malformed go.sum:\n%s:%d: wrong number of fields %v\n", file, lineno, len(f))
+			}
 		}
 		if f[2] == emptyGoModHash {
 			// Old bug; drop it.
@@ -530,7 +535,6 @@ func readGoSum(dst map[module.Version][]string, file string, data []byte) error
 		mod := module.Version{Path: f[0], Version: f[1]}
 		dst[mod] = append(dst[mod], f[2])
 	}
-	return nil
 }
 
 // HaveSum returns true if the go.sum file contains an entry for mod.

src/cmd/go/testdata/script/malformed_gosum_issue62345.txt

@@ -0,0 +1,51 @@
+! go mod download
+stderr '^malformed go.sum:\n.*go.sum:3: wrong number of fields 5\n$'
+
+go mod tidy
+cmp go.sum go.sum.after-tidy
+
+-- go.mod --
+module m
+
+go 1.20
+
+require rsc.io/quote v1.5.2
+
+require (
+	golang.org/x/text v0.0.0-20170915032832-14c0d48ead0c // indirect
+	rsc.io/sampler v1.3.0 // indirect
+	rsc.io/testonly v1.0.0 // indirect
+)
+
+-- go.sum --
+golang.org/x/text v0.0.0-20170915032832-14c0d48ead0c h1:pvCbr/wm8HzDD3fVywevekufpn6tCGPY3spdHeZJEsw=
+golang.org/x/text v0.0.0-20170915032832-14c0d48ead0c/go.mod h1:NqM8EUOU14njkJ3fqMW+pc6Ldnwhi/IjpwHt7yyuwOQ=
+rsc.io/quote v1.5.2 # invalid line
+rsc.io/quote v1.5.2/go.mod h1:LzX7hefJvL54yjefDEDHNONDjII0t9xZLPXsUe+TKr0=
+rsc.io/sampler v1.3.0 h1:HLGR/BgEtI3r0uymSP/nl2uPLsUnNJX8toRyhfpBTII=
+rsc.io/sampler v1.3.0/go.mod h1:T1hPZKmBbMNahiBKFy5HrXp6adAjACjK9JXDnKaTXpA=
+rsc.io/testonly v1.0.0 h1:K/VWHdO+Jv7woUXG0GzVNx1czBXUt3Ib1deaMn+xk64=
+rsc.io/testonly v1.0.0/go.mod h1:OqmGbIFOcF+XrFReLOGZ6BhMM7uMBiQwZsyNmh74SzY=
+
+-- main.go --
+package main
+
+import (
+	"fmt"
+
+	"rsc.io/quote"
+)
+
+func main() {
+	fmt.Println(quote.Hello())
+}
+
+-- go.sum.after-tidy --
+golang.org/x/text v0.0.0-20170915032832-14c0d48ead0c h1:pvCbr/wm8HzDD3fVywevekufpn6tCGPY3spdHeZJEsw=
+golang.org/x/text v0.0.0-20170915032832-14c0d48ead0c/go.mod h1:NqM8EUOU14njkJ3fqMW+pc6Ldnwhi/IjpwHt7yyuwOQ=
+rsc.io/quote v1.5.2 h1:3fEykkD9k7lYzXqCYrwGAf7iNhbk4yCjHmKBN9td4L0=
+rsc.io/quote v1.5.2/go.mod h1:LzX7hefJvL54yjefDEDHNONDjII0t9xZLPXsUe+TKr0=
+rsc.io/sampler v1.3.0 h1:HLGR/BgEtI3r0uymSP/nl2uPLsUnNJX8toRyhfpBTII=
+rsc.io/sampler v1.3.0/go.mod h1:T1hPZKmBbMNahiBKFy5HrXp6adAjACjK9JXDnKaTXpA=
+rsc.io/testonly v1.0.0 h1:K/VWHdO+Jv7woUXG0GzVNx1czBXUt3Ib1deaMn+xk64=
+rsc.io/testonly v1.0.0/go.mod h1:OqmGbIFOcF+XrFReLOGZ6BhMM7uMBiQwZsyNmh74SzY=