race condition in append

[notnmeyer]

two daylog -a invocations can race and corrupt data.

problem

Append reads the file, appends in memory, then writes back. if two processes write at the same time:

1. process a reads file
2. process b reads file
3. process a writes (a + content)
4. process b writes (a + different content) — overwrites a's entry

reproduction

for i in {1..10}; do daylog -a "entry $i" & done
wait
daylog show  # some entries missing

fix

use os.OpenFile with O_APPEND flag instead of read-modify-write:

f, err := os.OpenFile(d.Path, os.O_APPEND|os.O_WRONLY, 0644)
if err != nil {
    return err
}
defer f.Close()
_, err = f.WriteString(content)

atomic append means the kernel handles the ordering. no locking needed.