From: Michael Matloob <matloob@golang.org>
Date: Wed, 26 Feb 2025 21:31:41 +0000 (-0500)
Subject: cmd/go/internal/mmap: close file after mmap
X-Git-Tag: go1.25rc1~785
X-Git-Url: http://www.git.cypherpunks.su/?a=commitdiff_plain;h=2620cc1caae099c5947bfd5ff2c10bd4a3b628fd;p=gostls13.git

cmd/go/internal/mmap: close file after mmap

Closing the file after mmap will reduce the number of files associated
with the process. This will not likely help with #71698 but it doesn't
hurt to close the files and should simplify lsof output.

For #71698

Change-Id: I06a1bf91914afc7703783fe1a38d8bc5a6fb3d9d
Reviewed-on: https://go-review.googlesource.com/c/go/+/653055
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Alan Donovan <adonovan@google.com>
---

diff --git a/src/cmd/go/internal/cache/cache.go b/src/cmd/go/internal/cache/cache.go
index 26913dd959..23cc531e69 100644
--- a/src/cmd/go/internal/cache/cache.go
+++ b/src/cmd/go/internal/cache/cache.go
@@ -296,6 +296,10 @@ func GetBytes(c Cache, id ActionID) ([]byte, Entry, error) {
 // GetMmap looks up the action ID in the cache and returns
 // the corresponding output bytes.
 // GetMmap should only be used for data that can be expected to fit in memory.
+// The boolean result indicates whether the file was opened.
+// If it is true, the caller should avoid attempting
+// to write to the file on Windows, because Windows locks
+// the open file, and writes to it will fail.
 func GetMmap(c Cache, id ActionID) ([]byte, Entry, bool, error) {
 	entry, err := c.Get(id)
 	if err != nil {
diff --git a/src/cmd/go/internal/mmap/mmap.go b/src/cmd/go/internal/mmap/mmap.go
index fd374df82e..cd7ea80f2d 100644
--- a/src/cmd/go/internal/mmap/mmap.go
+++ b/src/cmd/go/internal/mmap/mmap.go
@@ -22,11 +22,30 @@ type Data struct {
 }
 
 // Mmap maps the given file into memory.
+// The boolean result indicates whether the file was opened.
+// If it is true, the caller should avoid attempting
+// to write to the file on Windows, because Windows locks
+// the open file, and writes to it will fail.
 func Mmap(file string) (Data, bool, error) {
 	f, err := os.Open(file)
 	if err != nil {
 		return Data{}, false, err
 	}
 	data, err := mmapFile(f)
+
+	// Closing the file causes it not to count against this process's
+	// limit on open files; however, the mapping still counts against
+	// the system-wide limit, which is typically higher. Examples:
+	//
+	//     macOS process (sysctl kern.maxfilesperproc):  61440
+	//     macOS system  (sysctl kern.maxfiles):        122880
+	//     linux process (ulimit -n)                   1048576
+	//     linux system  (/proc/sys/fs/file-max)        100000
+	if cerr := f.Close(); cerr != nil && err == nil {
+		return data, true, cerr
+	}
+
+	// The file is still considered to be in use on Windows after
+	// it's closed because of the mapping.
 	return data, true, err
 }