view tests/test-convert-svn-startrev.t @ 46667:93e9f448273c

rhg: Add support for automatic fallback to Python

`rhg` is a command-line application that can do a small subset of what
`hg` can. It is written entirely in Rust, which avoids the cost of starting
a Python interpreter and importing many Python modules.
In a script that runs many `hg` commands, this cost can add up.

However making users decide when to use `rhg` instead of `hg` is
not practical as we want the subset of supported functionality
to grow over time.

Instead we introduce "fallback" behavior where, when `rhg` encounters
something (a sub-command, a repository format, …) that is not implemented
in Rust-only, it does nothing but silently start a subprocess of
Python-based `hg` running the same command.
That way `rhg` becomes a drop-in replacement for `hg` that sometimes
goes faster. Whether Python is used should be an implementation detail
not apparent to users (other than through speed).

A new `fallback` value is added to the previously introduced
`rhg.on-unsupported` configuration key. When in this mode, the new
`rhg.fallback-executable` config is determine what command to use
to run a Python-based `hg`.

The previous `rhg.on-unsupported = abort-silent` configuration was designed
to let a wrapper script call `rhg` and then fall back to `hg` based on the
exit code. This is still available, but having fallback behavior built-in
in rhg might be easier for users instead of leaving that script "as an
exercise for the reader".

Using a subprocess like this is not idea, especially when `rhg` is to be
installed in `$PATH` as `hg`, since the other `hg.py` executable needs
to still be available… somewhere. Eventually this could be replaced
by using PyOxidizer to a have a single executable that embeds a Python
interpreter, but only starts it when needed.

Differential Revision: https://phab.mercurial-scm.org/D10093

#require svn svn-bindings

  $ cat >> $HGRCPATH <<EOF
  > [extensions]
  > convert =
  > EOF
  $ convert()
  > {
  >     startrev=$1
  >     repopath=A-r$startrev-hg
  >     hg convert --config convert.svn.startrev=$startrev \
  >         --config convert.svn.trunk=branches/branch1 \
  >         --config convert.svn.branches="  " \
  >         --config convert.svn.tags= \
  >         --datesort svn-repo $repopath
  >     hg -R $repopath log -G \
  >         --template '{rev} {desc|firstline} files: {files}\n'
  >     echo
  > }

  $ svnadmin create svn-repo
  $ svnadmin load -q svn-repo < "$TESTDIR/svn/startrev.svndump"

Convert before branching point

  $ convert 3
  initializing destination A-r3-hg repository
  scanning source...
  sorting...
  converting...
  3 removeb
  2 changeaa
  1 branch, changeaaa
  0 addc,changeaaaa
  o  3 addc,changeaaaa files: a c
  |
  o  2 branch, changeaaa files: a
  |
  o  1 changeaa files: a
  |
  o  0 removeb files: a
  
  

Convert before branching point

  $ convert 4
  initializing destination A-r4-hg repository
  scanning source...
  sorting...
  converting...
  2 changeaa
  1 branch, changeaaa
  0 addc,changeaaaa
  o  2 addc,changeaaaa files: a c
  |
  o  1 branch, changeaaa files: a
  |
  o  0 changeaa files: a
  
  

Convert at branching point

  $ convert 5
  initializing destination A-r5-hg repository
  scanning source...
  sorting...
  converting...
  1 branch, changeaaa
  0 addc,changeaaaa
  o  1 addc,changeaaaa files: a c
  |
  o  0 branch, changeaaa files: a
  
  

Convert last revision only

  $ convert 6
  initializing destination A-r6-hg repository
  scanning source...
  sorting...
  converting...
  0 addc,changeaaaa
  o  0 addc,changeaaaa files: a c