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cmd/go/internal/cache: avoid ioutil.WriteFile for writing cache entries

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ioutil.WriteFile always truncates the destination file to 0 before
writing, which is inappropriate for unsynchronized, idempotent,
fixed-size files such as the cache entry files here.

Instead, truncate the file only after writing it, so that a second
write will never (even temporarily!) remove the contents of a
preceding write.

Fixes #29667

Change-Id: I16a53ce79d8a23d23580511cb6abd062f54b65ef
Reviewed-on: https://go-review.googlesource.com/c/go/+/188157
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>
This commit is contained in:
Bryan C. Mills 2019-07-25 20:26:46 -04:00
parent 8c3040d768
commit 2fc7574aab
2 changed files with 64 additions and 2 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

25
src/cmd/go/internal/cache/cache.go vendored
View File

@ -322,7 +322,7 @@ func (c *Cache) putIndexEntry(id ActionID, out OutputID, size int64, allowVerify
// in verify mode we are double-checking that the cache entries
// are entirely reproducible. As just noted, this may be unrealistic
// in some cases but the check is also useful for shaking out real bugs.
entry := []byte(fmt.Sprintf("v1 %x %x %20d %20d\n", id, out, size, time.Now().UnixNano()))
entry := fmt.Sprintf("v1 %x %x %20d %20d\n", id, out, size, time.Now().UnixNano())
if verify && allowVerify {
old, err := c.get(id)
if err == nil && (old.OutputID != out || old.Size != size) {
@ -332,7 +332,28 @@ func (c *Cache) putIndexEntry(id ActionID, out OutputID, size int64, allowVerify
}
}
file := c.fileName(id, "a")
if err := ioutil.WriteFile(file, entry, 0666); err != nil {
// Copy file to cache directory.
mode := os.O_WRONLY | os.O_CREATE
f, err := os.OpenFile(file, mode, 0666)
if err != nil {
return err
}
_, err = f.WriteString(entry)
if err == nil {
// Truncate the file only *after* writing it.
// (This should be a no-op, but truncate just in case of previous corruption.)
//
// This differs from ioutil.WriteFile, which truncates to 0 *before* writing
// via os.O_TRUNC. Truncating only after writing ensures that a second write
// of the same content to the same file is idempotent, and does not — even
// temporarily! — undo the effect of the first write.
err = f.Truncate(int64(len(entry)))
}
if closeErr := f.Close(); err == nil {
err = closeErr
}
if err != nil {
// TODO(bcmills): This Remove potentially races with another go command writing to file.
// Can we eliminate it?
os.Remove(file)

41
src/cmd/go/testdata/script/mod_list_compiled_concurrent.txt vendored Normal file
View File

@ -0,0 +1,41 @@
env GO111MODULE=on
[short] skip
# Regression test for golang.org/issue/29667:
# spurious 'failed to cache compiled Go files' errors.
# This test failed reliably when run with -count=10
# on a Linux workstation.
env GOCACHE=$WORK/gocache
mkdir $GOCACHE
go list -json -compiled -test=false -export=false -deps=true -- . &
go list -json -compiled -test=false -export=false -deps=true -- . &
go list -json -compiled -test=false -export=false -deps=true -- . &
go list -json -compiled -test=false -export=false -deps=true -- . &
go list -json -compiled -test=false -export=false -deps=true -- . &
go list -json -compiled -test=false -export=false -deps=true -- . &
go list -json -compiled -test=false -export=false -deps=true -- . &
go list -json -compiled -test=false -export=false -deps=true -- . &
go list -json -compiled -test=false -export=false -deps=true -- . &
go list -json -compiled -test=false -export=false -deps=true -- . &
go list -json -compiled -test=false -export=false -deps=true -- . &
go list -json -compiled -test=false -export=false -deps=true -- . &
go list -json -compiled -test=false -export=false -deps=true -- . &
go list -json -compiled -test=false -export=false -deps=true -- . &
go list -json -compiled -test=false -export=false -deps=true -- . &
go list -json -compiled -test=false -export=false -deps=true -- . &
go list -json -compiled -test=false -export=false -deps=true -- . &
go list -json -compiled -test=false -export=false -deps=true -- . &
go list -json -compiled -test=false -export=false -deps=true -- . &
go list -json -compiled -test=false -export=false -deps=true -- . &
wait
-- go.mod --
module sandbox/bar
-- bar.go --
package bar
import "C"