Reimplement ioutil.ReadDir with a version that avoids calling lstat

ioutil.ReadDir returns []os.FileInfo, which contains information on
each entry in the directory that is only available by calling
os.Lstat on the entry.  Finder only the name and type (regular,
directory or symlink) of the files, which on Linux kernels >= 2.6.4
is available in the return values of syscall.Getdents.

Replace ioutil.ReadDir with a call that uses syscall.Getdents
directly and collects the type information from the result.

Testing with:
rm -f /tmp/db && strace -fc finder -names Android.mk,Android.bp,Blueprints,CleanSpec.mk,TEST_MAPPING -exclude-dirs .git,.repo -prune-files .out-dir,.find-ignore -db /tmp/db .

Before:
  7.01   52.688304          63    833398         1 lstat
  1.90   14.246644          68    210523           getdents64
  1.25    9.370471          90    104286         1 openat

After:
  3.48   12.201385         117    104286         1 openat
  3.06   10.729138          51    210523           getdents64
  1.70    5.951892          57    104283         1 lstat

Pros:
Avoids 729115 calls to lstat.

Cons:
Requires copying ~200 lines of finicky buffer parsing code.
Puts all getdents calls (and possibly fallback lstat calls) onto
a non-blocking file descriptor, which will cause it to block a
thread and not just a goroutine.
Only works on Linux and Darwin.

Bug: 70897635
Test: m checkbuild
Change-Id: Iab9f82c38c8675d0b73b4e90540bb9e4d2ee52c1

0 files changed, 0 insertions, 0 deletions