Description
Go 1.24.0, plan9/386
Certain (many) plan9 files get generated on demand and don't have a known length ahead of time. The same situation exists with e.g. Linux /proc or /sys files.
os.ReadFile fails to read some of them.
For example, my /net/iproute on plan9 has length 0, according to ls -l:
term% ls -l /net/iproute
--rw-rw-r-- I 0 network gelnda 0 Mar. 1 16:45 /net/iproute
But I can cat it, and wc says it has 1215 bytes:
term% wc /net/iproute
23 138 1215 /net/iproute
os.ReadFile, on the other hand, returns (511 bytes, nil) truncating it without any error.
What's happening is that os.ReadFile stats it, sees it has 0 length, assumes it's a special /proc-like file, starts with a minimum 512 byte buffer, and then issues a 512 byte read. The plan9 kernel returns with (511, nil). os.ReadFile then has 1 byte remaining between its capacity and length so it issues a one byte read to the kernel and the read returns (0, io.EOF).
Apparently that /proc-y special file doesn't like small buffers. It looks like the buffer size has to be at least large enough to fit one whole serialized route?
Perhaps os.ReadFile needs to say that at least for files that reported 0 length stat size at the beginning, we never issue a read system call with a buffer smaller than 512 bytes?
/cc @rsc @ianlancetaylor