Re: [PATCH v4 06/10] trace2: use system/global config for default trace2 settings

["SZEDER Gábor" <szeder.dev@gmail.com>]

On Mon, Apr 15, 2019 at 01:39:47PM -0700, Jeff Hostetler via GitGitGadget wrote:
> From: Jeff Hostetler <jeffhost@microsoft.com>
> 
> Teach git to read the system and global config files for
> default Trace2 settings.  This allows system-wide Trace2 settings to
> be installed and inherited to make it easier to manage a collection of
> systems.
> 
> The original GIT_TR2* environment variables are loaded afterwards and
> can be used to override the system settings.
> 
> Only the system and global config files are used.  Repo and worktree
> local config files are ignored.  Likewise, the "-c" command line
> arguments are also ignored.  These limits are for performance reasons.
> 
> (1) For users not using Trace2, there should be minimal overhead to
> detect that Trace2 is not enabled.  In particular, Trace2 should not
> allocate lots of otherwise unused data strucutres.
> 
> (2) For accurate performance measurements, Trace2 should be initialized
> as early in the git process as possible, and before most of the normal
> git process initialization (which involves discovering the .git directory
> and reading a hierarchy of config files).

Reading the configuration that early causes unexpected and undesired
behavior change:

  $ sudo chmod a-rwx /usr/local/etc/gitconfig
  $ ./BUILDS/v2.21.0/bin/git
  usage: git [--version] [--help] [-C <path>] [-c <name>=<value>]
  <... snip rest of usage ...>
  $ strace ./BUILDS/v2.21.0/bin/git 2>&1 |grep config -c
  0
  $ ./git
  fatal: unable to access '/usr/local/etc/gitconfig': Permission denied
  $ ./git --version
  fatal: unable to access '/usr/local/etc/gitconfig': Permission denied

I think at least 'git', 'git --help', and 'git --version' should Just
Work, no matter what.


This breaks the 32 bit Linux build job on Travis CI, because:

  - In the 32 bit Docker image we change UID from root to regular user
    while preserving the environment, including $HOME.
    
  - Since $HOME is the default build prefix, Git will look for the
    system-wide configuration under '/root/etc/gitconfig', which fails
    as a regular user.

  - Our test harness checks early (i.e. earlier than setting
    GIT_CONFIG_NOSYSTEM=1) whether Git has been built successfully by
    attempting to run '$GIT_BUILD_DIR}/git', which fails because of
    the inaccessible system-wide config file, and in turn the harness
    assumes that Git hasn't been built and aborts.

  https://travis-ci.org/git/git/jobs/524403682#L1258