clone: warn when streaming was requested but couldn't be performed

This helps both users and the people who support them figure out why
a stream clone couldn't be performed.

In an upcoming patch we're going to add a way for servers to hard
abort on a full getbundle. In those cases servers might expect
clients to perform a stream clone, so it's important to communicate
why one couldn't be done.

mercurial/streamclone.py

@@ -80,11 +80,21 b' def canperformstreamclone(pullop, bailif'
         streamreqs = remote.capable('streamreqs')
         # This is weird and shouldn't happen with modern servers.
         if not streamreqs:
+            pullop.repo.ui.warn(_(
+                'warning: stream clone requested but server has them '
+                'disabled\n'))
             return False, None
 
         streamreqs = set(streamreqs.split(','))
         # Server requires something we don't support. Bail.
-        if streamreqs - repo.supportedformats:
+        missingreqs = streamreqs - repo.supportedformats
+        if missingreqs:
+            pullop.repo.ui.warn(_(
+                'warning: stream clone requested but client is missing '
+                'requirements: %s\n') % ', '.join(sorted(missingreqs)))
+            pullop.repo.ui.warn(
+                _('(see https://www.mercurial-scm.org/wiki/MissingRequirement '
+                  'for more information)\n'))
             return False, None
         requirements = streamreqs
 

tests/test-http-bundle1.t

@@ -58,6 +58,7 b' clone via stream'
 try to clone via stream, should use pull instead
 
   $ hg clone --uncompressed http://localhost:$HGPORT1/ copy2
+  warning: stream clone requested but server has them disabled
   requesting all changes
   adding changesets
   adding manifests
@@ -75,6 +76,8 b' try to clone via stream but missing requ'
   > EOF
 
   $ hg clone --config extensions.rsf=$TESTTMP/removesupportedformat.py --uncompressed http://localhost:$HGPORT/ copy3
+  warning: stream clone requested but client is missing requirements: generaldelta
+  (see https://www.mercurial-scm.org/wiki/MissingRequirement for more information)
   requesting all changes
   adding changesets
   adding manifests

tests/test-http.t

@@ -49,6 +49,7 b' clone via stream'
 try to clone via stream, should use pull instead
 
   $ hg clone --uncompressed http://localhost:$HGPORT1/ copy2
+  warning: stream clone requested but server has them disabled
   requesting all changes
   adding changesets
   adding manifests
@@ -66,6 +67,8 b' try to clone via stream but missing requ'
   > EOF
 
   $ hg clone --config extensions.rsf=$TESTTMP/removesupportedformat.py --uncompressed http://localhost:$HGPORT/ copy3
+  warning: stream clone requested but client is missing requirements: generaldelta
+  (see https://www.mercurial-scm.org/wiki/MissingRequirement for more information)
   requesting all changes
   adding changesets
   adding manifests