view tests/test-diffdir.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

  $ hg init
  $ touch a
  $ hg add a
  $ hg ci -m "a"

  $ echo 123 > b
  $ hg add b
  $ hg diff --nodates
  diff -r 3903775176ed b
  --- /dev/null
  +++ b/b
  @@ -0,0 +1,1 @@
  +123

  $ hg diff --nodates -r tip
  diff -r 3903775176ed b
  --- /dev/null
  +++ b/b
  @@ -0,0 +1,1 @@
  +123

  $ echo foo > a
  $ hg diff --nodates
  diff -r 3903775176ed a
  --- a/a
  +++ b/a
  @@ -0,0 +1,1 @@
  +foo
  diff -r 3903775176ed b
  --- /dev/null
  +++ b/b
  @@ -0,0 +1,1 @@
  +123

  $ hg diff -r ""
  hg: parse error: empty query
  [10]
  $ hg diff -r tip -r ""
  hg: parse error: empty query
  [10]

Remove a file that was added via merge. Since the file is not in parent 1,
it should not be in the diff.

  $ hg ci -m 'a=foo' a
  $ hg co -Cq null
  $ echo 123 > b
  $ hg add b
  $ hg ci -m "b"
  created new head
  $ hg merge 1
  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
  (branch merge, don't forget to commit)
  $ hg rm -f a
  $ hg diff --nodates

Rename a file that was added via merge. Since the rename source is not in
parent 1, the diff should be relative to /dev/null

  $ hg co -Cq 2
  $ hg merge 1
  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
  (branch merge, don't forget to commit)
  $ hg mv a a2
  $ hg diff --nodates
  diff -r cf44b38435e5 a2
  --- /dev/null
  +++ b/a2
  @@ -0,0 +1,1 @@
  +foo
  $ hg diff --nodates --git
  diff --git a/a2 b/a2
  new file mode 100644
  --- /dev/null
  +++ b/a2
  @@ -0,0 +1,1 @@
  +foo