fix: fix handling of merge commits by using overlayworkingctx

Most of this code was conceptually copied from what rebase does, with one small
difference: hgext.rebaserev.rebase uses branchmerge=True, while I had to use
branchmerge=False, or else it got really confused about updating to the same
revision in some situations. I believe that the difference is that rebase is
always dealing with *some* form of update - it never gets to mergemod.update if
the source and destination are the same, while we can encounter that situation
with fix. This may imply that this code has some issues with named branches that
should be investigated.

Differential Revision: https://phab.mercurial-scm.org/D7703

hgext/fix.py

@@ -730,36 +730,40 b' def replacerev(ui, repo, ctx, filedata, '
     ):
         return
 
-    def filectxfn(repo, memctx, path):
-        if path not in ctx:
-            return None
-        fctx = ctx[path]
-        copysource = fctx.copysource()
-        return context.memfilectx(
-            repo,
-            memctx,
-            path=fctx.path(),
-            data=filedata.get(path, fctx.data()),
-            islink=fctx.islink(),
-            isexec=fctx.isexec(),
-            copysource=copysource,
-        )
-
     extra = ctx.extra().copy()
     extra[b'fix_source'] = ctx.hex()
 
-    memctx = context.memctx(
+    wctx = context.overlayworkingctx(repo)
+    wctx.setbase(repo[newp1node])
+    merge.update(
         repo,
-        parents=(newp1node, newp2node),
+        ctx.rev(),
+        branchmerge=False,
+        force=True,
+        ancestor=p1rev,
+        mergeancestor=False,
+        wc=wctx,
+    )
+    copies.duplicatecopies(
+        repo, wctx, ctx.rev(), ctx.p1().rev(), skiprev=newp1node
+    )
+
+    for path in filedata.keys():
+        fctx = ctx[path]
+        copysource = fctx.copysource()
+        wctx.write(path, filedata[path], flags=fctx.flags())
+        if copysource:
+            wctx.markcopied(path, copysource)
+
+    memctx = wctx.tomemctx(
         text=ctx.description(),
-        files=set(ctx.files()) | set(filedata.keys()),
-        filectxfn=filectxfn,
-        user=ctx.user(),
+        branch=ctx.branch(),
+        extra=extra,
         date=ctx.date(),
-        extra=extra,
-        branch=ctx.branch(),
-        editor=None,
+        parents=(newp1node, newp2node),
+        user=ctx.user(),
     )
+
     sucnode = memctx.commit()
     prenode = ctx.node()
     if prenode == sucnode:

tests/test-fix.t

@@ -1456,3 +1456,242 b' changes.'
   2 through 2
 
   $ cd ..
+
+Test various cases around merges. We were previously dropping files if they were
+created on only the p2 side of the merge, so let's test permutations of:
+*   added, was fixed
+*   added, considered for fixing but was already good
+*   added, not considered for fixing
+*   modified, was fixed
+*   modified, considered for fixing but was already good
+*   modified, not considered for fixing
+
+Before the bug was fixed where we would drop files, this test demonstrated the
+following issues:
+*   new_in_r1.ignored, new_in_r1_already_good.changed, and
+>   mod_in_r1_already_good.changed were NOT in the manifest for the merge commit
+*   mod_in_r1.ignored had its contents from r0, NOT r1.
+
+We're also setting a named branch for every commit to demonstrate that the
+branch is kept intact and there aren't issues updating to another branch in the
+middle of fix.
+
+  $ hg init merge_keeps_files
+  $ cd merge_keeps_files
+  $ for f in r0 mod_in_r1 mod_in_r2 mod_in_merge mod_in_child; do
+  >   for c in changed whole ignored; do
+  >     printf "hello\n" > $f.$c
+  >   done
+  >   printf "HELLO\n" > "mod_in_${f}_already_good.changed"
+  > done
+  $ hg branch -q r0
+  $ hg ci -Aqm 'r0'
+  $ hg phase -p
+  $ make_test_files() {
+  >   printf "world\n" >> "mod_in_$1.changed"
+  >   printf "world\n" >> "mod_in_$1.whole"
+  >   printf "world\n" >> "mod_in_$1.ignored"
+  >   printf "WORLD\n" >> "mod_in_$1_already_good.changed"
+  >   printf "new in $1\n" > "new_in_$1.changed"
+  >   printf "new in $1\n" > "new_in_$1.whole"
+  >   printf "new in $1\n" > "new_in_$1.ignored"
+  >   printf "ALREADY GOOD, NEW IN THIS REV\n" > "new_in_$1_already_good.changed"
+  > }
+  $ make_test_commit() {
+  >   make_test_files "$1"
+  >   hg branch -q "$1"
+  >   hg ci -Aqm "$2"
+  > }
+  $ make_test_commit r1 "merge me, pt1"
+  $ hg co -q ".^"
+  $ make_test_commit r2 "merge me, pt2"
+  $ hg merge -qr 1
+  $ make_test_commit merge "evil merge"
+  $ make_test_commit child "child of merge"
+  $ make_test_files wdir
+  $ hg fix -r 'not public()' -w
+  $ hg log -G -T'{rev}:{shortest(node,8)}: branch:{branch} desc:{desc}'
+  @  8:c22ce900: branch:child desc:child of merge
+  |
+  o    7:5a30615a: branch:merge desc:evil merge
+  |\
+  | o  6:4e5acdc4: branch:r2 desc:merge me, pt2
+  | |
+  o |  5:eea01878: branch:r1 desc:merge me, pt1
+  |/
+  o  0:0c548d87: branch:r0 desc:r0
+  
+  $ hg files -r tip
+  mod_in_child.changed
+  mod_in_child.ignored
+  mod_in_child.whole
+  mod_in_child_already_good.changed
+  mod_in_merge.changed
+  mod_in_merge.ignored
+  mod_in_merge.whole
+  mod_in_merge_already_good.changed
+  mod_in_mod_in_child_already_good.changed
+  mod_in_mod_in_merge_already_good.changed
+  mod_in_mod_in_r1_already_good.changed
+  mod_in_mod_in_r2_already_good.changed
+  mod_in_r0_already_good.changed
+  mod_in_r1.changed
+  mod_in_r1.ignored
+  mod_in_r1.whole
+  mod_in_r1_already_good.changed
+  mod_in_r2.changed
+  mod_in_r2.ignored
+  mod_in_r2.whole
+  mod_in_r2_already_good.changed
+  new_in_child.changed
+  new_in_child.ignored
+  new_in_child.whole
+  new_in_child_already_good.changed
+  new_in_merge.changed
+  new_in_merge.ignored
+  new_in_merge.whole
+  new_in_merge_already_good.changed
+  new_in_r1.changed
+  new_in_r1.ignored
+  new_in_r1.whole
+  new_in_r1_already_good.changed
+  new_in_r2.changed
+  new_in_r2.ignored
+  new_in_r2.whole
+  new_in_r2_already_good.changed
+  r0.changed
+  r0.ignored
+  r0.whole
+  $ for f in "$(hg files -r tip)"; do hg cat -r tip $f -T'{path}:\n{data}\n'; done
+  mod_in_child.changed:
+  hello
+  WORLD
+  
+  mod_in_child.ignored:
+  hello
+  world
+  
+  mod_in_child.whole:
+  HELLO
+  WORLD
+  
+  mod_in_child_already_good.changed:
+  WORLD
+  
+  mod_in_merge.changed:
+  hello
+  WORLD
+  
+  mod_in_merge.ignored:
+  hello
+  world
+  
+  mod_in_merge.whole:
+  HELLO
+  WORLD
+  
+  mod_in_merge_already_good.changed:
+  WORLD
+  
+  mod_in_mod_in_child_already_good.changed:
+  HELLO
+  
+  mod_in_mod_in_merge_already_good.changed:
+  HELLO
+  
+  mod_in_mod_in_r1_already_good.changed:
+  HELLO
+  
+  mod_in_mod_in_r2_already_good.changed:
+  HELLO
+  
+  mod_in_r0_already_good.changed:
+  HELLO
+  
+  mod_in_r1.changed:
+  hello
+  WORLD
+  
+  mod_in_r1.ignored:
+  hello
+  world
+  
+  mod_in_r1.whole:
+  HELLO
+  WORLD
+  
+  mod_in_r1_already_good.changed:
+  WORLD
+  
+  mod_in_r2.changed:
+  hello
+  WORLD
+  
+  mod_in_r2.ignored:
+  hello
+  world
+  
+  mod_in_r2.whole:
+  HELLO
+  WORLD
+  
+  mod_in_r2_already_good.changed:
+  WORLD
+  
+  new_in_child.changed:
+  NEW IN CHILD
+  
+  new_in_child.ignored:
+  new in child
+  
+  new_in_child.whole:
+  NEW IN CHILD
+  
+  new_in_child_already_good.changed:
+  ALREADY GOOD, NEW IN THIS REV
+  
+  new_in_merge.changed:
+  NEW IN MERGE
+  
+  new_in_merge.ignored:
+  new in merge
+  
+  new_in_merge.whole:
+  NEW IN MERGE
+  
+  new_in_merge_already_good.changed:
+  ALREADY GOOD, NEW IN THIS REV
+  
+  new_in_r1.changed:
+  NEW IN R1
+  
+  new_in_r1.ignored:
+  new in r1
+  
+  new_in_r1.whole:
+  NEW IN R1
+  
+  new_in_r1_already_good.changed:
+  ALREADY GOOD, NEW IN THIS REV
+  
+  new_in_r2.changed:
+  NEW IN R2
+  
+  new_in_r2.ignored:
+  new in r2
+  
+  new_in_r2.whole:
+  NEW IN R2
+  
+  new_in_r2_already_good.changed:
+  ALREADY GOOD, NEW IN THIS REV
+  
+  r0.changed:
+  hello
+  
+  r0.ignored:
+  hello
+  
+  r0.whole:
+  hello
+