bug#33129: 26.1; `vc-hg-find-file-hook' can never detect files with conflicts

bug#33129: 26.1; `vc-hg-find-file-hook' can never detect files with conflicts

[Daniel Pittman]

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The `vc-hg-find-file-hook' tries to automatically invoke `smerge-mode'
when a file with conflicts is visited.  Unfortunately, due to incorrect
assumptions, it rarely -- possibly never -- actually manages to detect
the files.

There are two incorrect assumptions in the code that prevent this:

First, that `vc-state' will already be cached by the backend, which is
not true in at least the cases of visiting the file, reverting the file,
or using `vc-refresh-state'.

It is certainly possible that in some case or other the hook works, this
assumption means that it is almost never going to behave as intended.

Second, it assumes that a ".orig" backup file is created by Mercurial
next to the current file, which is only true by default: these files can
be redirected to an alternate path, or disabled entirely, by the user.

This is especially common when working on a network file-system, since
additional file operations can slow down the Mercurial merge process, or
when users have some other mechanism to handle the failure case.

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bug#33129: Acknowledgement (26.1; `vc-hg-find-file-hook' can never detect files with conflicts)

[Daniel Pittman]

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Please find attached a patch fixing both the issues identified in this bug:

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From ade637fb894633fdfc1ea6ba54e4b31d26ec028e Mon Sep 17 00:00:00 2001
From: Daniel Pittman <slippycheeze@google.com>
Date: Tue, 23 Oct 2018 11:24:04 -0400
Subject: [PATCH] Fix interaction between vc-hg find-file-hook and vc state
 caching

Bad assumptions in the `vc-hg-find-file-hook' prevented it from
working. This correctly them.  (Bug#33129).

2018-10-23  Daniel Pittman  <slippycheeze@google.com>

	* lisp/vc/vc-hg.el (vc-hg-find-file-hook): This function made two
	assumptions about conflicted files that were not accurate,
	preventing conflicts in files ever being detected.

	The first was that the `vc-state' was cache by the time this was
	invoked, which it is not - at least when visiting the file, or
	using `vc-refresh-state'.

	The second was that a file with the ".orig" extension would be
	present, next to the file being visited.  This is the default
	behavior of Mercurial, but can be overridden by the user.

	Since the VC mode-line code will shortly calculate the state for
	display, the optimization of testing for the ".orig" file only
	delayed this work by a few moments.
---
 etc/NEWS         | 6 ++++++
 lisp/vc/vc-hg.el | 4 +---
 2 files changed, 7 insertions(+), 3 deletions(-)

diff --git a/etc/NEWS b/etc/NEWS
index 7a98b492f1..e6b628f97d 100644
--- a/etc/NEWS
+++ b/etc/NEWS
@@ -340,6 +340,12 @@ When no files are marked, all modified files are stashed, as before.
 
 *** The new hook 'vc-retrieve-tag-hook' runs after retrieving a tag.
 
+--- `vc-hg' now invokes `smerge-mode' correctly when visiting files.
+Code that attempted to invoke `smerge-mode' when visiting an Hg file
+with conflicts existed in earlier versions of Emacs, but incorrectly
+never detected a conflict due to invalid assumptions about cached
+values.
+
 ** diff-mode
 *** Hunks are now automatically refined by default.
 To disable it, set the new defcustom 'diff-font-lock-refine' to nil.
diff --git a/lisp/vc/vc-hg.el b/lisp/vc/vc-hg.el
index 3696573595..d528813bc0 100644
--- a/lisp/vc/vc-hg.el
+++ b/lisp/vc/vc-hg.el
@@ -1142,11 +1142,9 @@ REV is the revision to check out into WORKFILE."
 
 (defun vc-hg-find-file-hook ()
   (when (and buffer-file-name
-             (file-exists-p (concat buffer-file-name ".orig"))
              ;; Hg does not seem to have a "conflict" status, eg
              ;; hg http://bz.selenic.com/show_bug.cgi?id=2724
-             (memq (vc-file-getprop buffer-file-name 'vc-state)
-                   '(edited conflict))
+             (memq (vc-state buffer-file-name) '(edited conflict))
              ;; Maybe go on to check that "hg resolve -l" says "U"?
              ;; If "hg resolve -l" says there's a conflict but there are no
              ;; conflict markers, it's not clear what we should do.
-- 
2.19.1

bug#33129: Acknowledgement (26.1; `vc-hg-find-file-hook' can never detect files with conflicts)

[Eli Zaretskii]

> From: Daniel Pittman <slippycheeze@google.com>
> Date: Tue, 23 Oct 2018 12:10:35 -0400
> 
> Please find attached a patch fixing both the issues identified in this bug:

Thanks, pushed to the master branch.