os: race between File.Close, File.Write, and other Opens

[rsc]

If you have a goroutine calling f.Close at the same time another calls f.Write, it can
happen that the f.Close happens, then an unrelated os.Open reuses the fd, then the
f.Write writes to the wrong fd. The timing would have to be just right, since Close does
set f.fd = -1, but it could happen.  We could decide this is not worth protecting
against, or we could introduce an rwlock on the File that must be rlocked across all
fd-using method implementations and wlocked by Close.

Investigate for 1.4.

Related to issue #5792.

[griesemer]

Comment 1:

Labels changed: added repo-main.

[rsc]

Comment 2:

Too late to make such a fundamental change.

Labels changed: added release-go1.5, removed release-go1.4.

[larzconwell]

If we do decide to add a rw mutex I've added the following CL that implements that in os.file https://go-review.googlesource.com/#/c/10186/

[gopherbot]

CL https://golang.org/cl/10186 mentions this issue.

[ianlancetaylor]

Using an RWMutex for file.fd will mean that if a Read hangs in the kernel waiting for data, a Close on the same file will hang waiting for the read to complete. I don't think that is desirable.

I think we should postpone this until issue #6817 is fixed.