templater: replace templatepath() with function that also opens the file

For frozen binaries, such as those created by PyOxidizer, I plan to
make it so the templatespec can keep an opened file/resource to read
from instead of needing a file path. Having `templatepath()` return an
opened file should help with that. At this point, it's just a wasteful
extra opening of mapfiles that we'll open again later. I'll update the
read-side next so it reads from the file-like object without opening
the file again.

Differential Revision: https://phab.mercurial-scm.org/D8892

mercurial/debugcommands.py

@@ -1672,7 +1672,7 b' def debuginstall(ui, **opts):'
     fm.write(b'templatedirs', b'checking templates (%s)...\n', p or b'')
     fm.condwrite(not p, b'', _(b" no template directories found\n"))
     if p:
-        m = templater.templatepath(b"map-cmdline.default")
+        (m, fp) = templater.open_template(b"map-cmdline.default")
         if m:
             # template found, check if it is working
             err = None

mercurial/formatter.py

@@ -599,9 +599,9 b' def lookuptemplate(ui, topic, tmpl):'
 
     # perhaps a stock style?
     if not os.path.split(tmpl)[0]:
-        mapname = templater.templatepath(
+        (mapname, fp) = templater.open_template(
             b'map-cmdline.' + tmpl
-        ) or templater.templatepath(tmpl)
+        ) or templater.open_template(tmpl)
         if mapname:
             return mapfile_templatespec(topic, mapname)
 

mercurial/logcmdutil.py

@@ -627,9 +627,9 b' def _lookuptemplate(ui, tmpl, style):'
     if not tmpl and style:
         mapfile = style
         if not os.path.split(mapfile)[0]:
-            mapname = templater.templatepath(
+            (mapname, fp) = templater.open_template(
                 b'map-cmdline.' + mapfile
-            ) or templater.templatepath(mapfile)
+            ) or templater.open_template(mapfile)
             if mapname:
                 mapfile = mapname
         return formatter.mapfile_templatespec(b'changeset', mapfile)

mercurial/templater.py

@@ -1071,12 +1071,15 b' def templatedir():'
     return path if os.path.isdir(path) else None
 
 
-def templatepath(name):
-    '''return location of template file. returns None if not found.'''
-    dir = templatedir()
-    if dir is None:
-        return None
-    f = os.path.join(templatedir(), name)
-    if f and os.path.isfile(f):
-        return f
-    return None
+def open_template(name):
+    '''returns a file-like object for the given template, and its full path'''
+    templatepath = templatedir()
+    if templatepath is not None or os.path.isabs(name):
+        f = os.path.join(templatepath, name)
+        try:
+            return f, open(f, mode='rb')
+        except EnvironmentError:
+            return None, None
+    else:
+        # TODO: read from resources here
+        return None, None