os: document that WriteFile is not atomic

Fixes #56173

Change-Id: I03a3ad769c99c0bdb78b1d757173d630879fd4dd
GitHub-Last-Rev: e3e31fa
GitHub-Pull-Request: #56282
Reviewed-on: https://go-review.googlesource.com/c/go/+/443495
TryBot-Result: Gopher Robot <[email protected]>
Reviewed-by: Michael Knyszek <[email protected]>
Reviewed-by: Rob Pike <[email protected]>
Reviewed-by: Ian Lance Taylor <[email protected]>
Auto-Submit: Ian Lance Taylor <[email protected]>
Run-TryBot: Ian Lance Taylor <[email protected]>

Author: RichardoC

src/os/file.go

@@ -710,6 +710,8 @@ func ReadFile(name string) ([]byte, error) {
 // WriteFile writes data to the named file, creating it if necessary.
 // If the file does not exist, WriteFile creates it with permissions perm (before umask);
 // otherwise WriteFile truncates it before writing, without changing permissions.
+// Since Writefile requires multiple system calls to complete, a failure mid-operation
+// can leave the file in a partially written state.
 func WriteFile(name string, data []byte, perm FileMode) error {
 	f, err := OpenFile(name, O_WRONLY|O_CREATE|O_TRUNC, perm)
 	if err != nil {