fileset: detect unintentional existing() invocation at runtime A fileset predicate can invoke 'matchctx.existing()' successfully, even if it isn't marked as "existing caller". It is aborted only in some corner cases: e.g. there were one deleted file in the working directory (see 8a0513bf030a for detail). This patch makes 'matchctx.existing()' invocation abort if not '_existingenabled', which is true only while "existing caller" running. After this changes, non-"existing caller" predicate function is aborted immediately, whenever it invokes 'matchctx.existing()'. This prevent developer from forgetting to mark a predicate as "existing caller". BTW, unintentional 'matchctx.status()' invocation can be detected easily without any additional trick like this patch, because it returns 'None' if a predicate isn't marked as "status caller", and referring field (e.g. '.modified') of it is always aborted.

--- a/tests/test-fileset.t	Mon Dec 21 22:31:16 2015 +0900
+++ b/tests/test-fileset.t	Mon Dec 21 22:31:16 2015 +0900
@@ -328,3 +328,22 @@
   b2link
   bin
   c1
+
+Test detection of unintentional 'matchctx.existing()' invocation
+
+  $ cat > $TESTTMP/existingcaller.py <<EOF
+  > from mercurial import fileset
+  > 
+  > @fileset.predicate('existingcaller()', callexisting=False)
+  > def existingcaller(mctx, x):
+  >     # this 'mctx.existing()' invocation is unintentional
+  >     return [f for f in mctx.existing()]
+  > EOF
+
+  $ cat >> .hg/hgrc <<EOF
+  > [extensions]
+  > existingcaller = $TESTTMP/existingcaller.py
+  > EOF
+
+  $ fileset 'existingcaller()' 2>&1 | tail -1
+  AssertionError: unexpected existing() invocation