cmd/internal/diff: skip over Cygwin warning in diff output

This happens on Windows. Don't let it stop us.

Change-Id: Ie2115d5825e1c2217f237ed373adb35594a5aaff
Reviewed-on: https://go-review.googlesource.com/c/go/+/293850
Trust: Russ Cox <rsc@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>

diff --git a/src/cmd/internal/diff/diff.go b/src/cmd/internal/diff/diff.go
index c0ca2f310630cd7266a770d6b42748ae034cb050..0ec2d7f8f95dceb7d5872b842e650c040d24fbc4 100644 (file)
--- a/src/cmd/internal/diff/diff.go
+++ b/src/cmd/internal/diff/diff.go
@@ -7,6 +7,7 @@
 package diff
 
 import (
+       "bytes"
        exec "internal/execabs"
        "io/ioutil"
        "os"
@@ -38,6 +39,25 @@ func Diff(prefix string, b1, b2 []byte) ([]byte, error) {
                // Ignore that failure as long as we get output.
                err = nil
        }
+
+       // If we are on Windows and the diff is Cygwin diff,
+       // machines can get into a state where every Cygwin
+       // command works fine but prints a useless message like:
+       //
+       //      Cygwin WARNING:
+       //        Couldn't compute FAST_CWD pointer.  This typically occurs if you're using
+       //        an older Cygwin version on a newer Windows.  Please update to the latest
+       //        available Cygwin version from https://cygwin.com/.  If the problem persists,
+       //        please see https://cygwin.com/problems.html
+       //
+       // Skip over that message and just return the actual diff.
+       if len(data) > 0 && !bytes.HasPrefix(data, []byte("--- ")) {
+               i := bytes.Index(data, []byte("\n--- "))
+               if i >= 0 && i < 80*10 && bytes.Contains(data[:i], []byte("://cygwin.com/")) {
+                       data = data[i+1:]
+               }
+       }
+
        return data, err
 }