phases: check HG_PENDING strictly

Before this patch, checking HG_PENDING in phases.py might cause
unintentional reading unrelated 'phaseroots.pending' in, because it
just examines existence of HG_PENDING environment variable.

This patch uses txnutil.trypending() to check HG_PENDING strictly.

BTW, this patch may cause failure of bisect in the repository of
Mercurial itself, if examination at bisecting assumes that an external
hook can see all pending changes while nested transactions across
repositories.

This invisibility issue will be fixed by subsequent patch, which
allows HG_PENDING to refer multiple repositories.

mercurial/phases.py

@@ -113,9 +113,9 b' from .node import ('
     short,
 )
 from . import (
-    encoding,
     error,
     smartset,
+    txnutil,
 )
 
 allphases = public, draft, secret = range(3)
@@ -137,15 +137,7 b' def _readroots(repo, phasedefaults=None)'
     dirty = False
     roots = [set() for i in allphases]
     try:
-        f = None
-        if 'HG_PENDING' in encoding.environ:
-            try:
-                f = repo.svfs('phaseroots.pending')
-            except IOError as inst:
-                if inst.errno != errno.ENOENT:
-                    raise
-        if f is None:
-            f = repo.svfs('phaseroots')
+        f, pending = txnutil.trypending(repo.root, repo.svfs, 'phaseroots')
         try:
             for line in f:
                 phase, nh = line.split()

tests/test-phases.t

@@ -590,3 +590,47 b' because repo.cancopy() is False'
   crosschecking files in changesets and manifests
   checking files
   7 files, 8 changesets, 7 total revisions
+
+  $ cd ..
+
+check whether HG_PENDING makes pending changes only in related
+repositories visible to an external hook.
+
+(emulate a transaction running concurrently by copied
+.hg/phaseroots.pending in subsequent test)
+
+  $ cat > $TESTTMP/savepending.sh <<EOF
+  > cp .hg/store/phaseroots.pending  .hg/store/phaseroots.pending.saved
+  > exit 1 # to avoid changing phase for subsequent tests
+  > EOF
+  $ cd push-dest
+  $ hg phase 6
+  6: draft
+  $ hg --config hooks.pretxnclose="sh $TESTTMP/savepending.sh" phase -f -s 6
+  transaction abort!
+  rollback completed
+  abort: pretxnclose hook exited with status 1
+  [255]
+  $ cp .hg/store/phaseroots.pending.saved .hg/store/phaseroots.pending
+
+(check (in)visibility of phaseroot while transaction running in repo)
+
+  $ cat > $TESTTMP/checkpending.sh <<EOF
+  > echo '@initialrepo'
+  > hg -R $TESTTMP/initialrepo phase 7
+  > echo '@push-dest'
+  > hg -R $TESTTMP/push-dest phase 6
+  > exit 1 # to avoid changing phase for subsequent tests
+  > EOF
+  $ cd ../initialrepo
+  $ hg phase 7
+  7: public
+  $ hg --config hooks.pretxnclose="sh $TESTTMP/checkpending.sh" phase -f -s 7
+  @initialrepo
+  7: secret
+  @push-dest
+  6: draft
+  transaction abort!
+  rollback completed
+  abort: pretxnclose hook exited with status 1
+  [255]