[SR-604] llbuild should understand symbolic links more precisely

[ddunbar]

Previous ID	SR-604
Radar	None
Original Reporter	@ddunbar
Type	Bug

Additional Detail from JIRA

Votes	0
Component/s	llbuild
Labels	Bug
Assignee	None
Priority	Medium

md5: d36aca20d0c383ff9b730a94e12b72ea

Issue Description:

In the following example, it isn't clear that llbuild should rebuild the link on the second iteration – the semantics of the build file are such that the ln is up to date, but because it doesn't exist, and we `stat()`, then we rebuild

$ cat t.llbuild 
client:
  name: basic
  version: 0

targets:
  "": ["foo-stat"]

commands:
  C1:
    tool: shell
    outputs: ["foo"]
    args: ln -sf does/not/exist foo

  C2:
    tool: shell
    inputs: ["foo"]
    outputs: ["foo-stat"]
    args: stat -s foo | tee foo-stat

$ llbuild buildsystem build -f t.llbuild 
/bin/sh -c "ln -sf does/not/exist foo"
/bin/sh -c "stat -s foo | tee foo-stat"
st_dev=16777225 st_ino=14250438 st_mode=0120755 st_nlink=1 st_uid=501 st_gid=0 st_rdev=0 st_size=14 st_atime=1453531949 st_mtime=1453531949 st_ctime=1453531949 st_birthtime=1453531949 st_blksize=4096 st_blocks=8 st_flags=0

$ llbuild buildsystem build -f t.llbuild 
/bin/sh -c "ln -sf does/not/exist foo"

We should probably have a way for the client to specify the appropriate semantics to use for file based nodes. It isn't clear to me yet whether that should be an attribute tied to the file or the command, though.