[PATCH] mail: take --encoding and HGENCODING into account

[Gábor Stefanik]

# HG changeset patch
# User Gábor Stefanik <gabor.stefanik at nng.com>
# Date 1475667922 -7200
#      Wed Oct 05 13:45:22 2016 +0200
# Node ID 31350841be0c6af1c335fb02b28b8fd1f79089b9
# Parent  91a3c58ecf938ed675f5364b88f0d663f12b0047
mail: take --encoding and HGENCODING into account

Fall back to our encoding strategy for sending MIME text
that's neither ASCII nor UTF-8.

diff --git a/mercurial/mail.py b/mercurial/mail.py
--- a/mercurial/mail.py
+++ b/mercurial/mail.py
@@ -205,22 +205,40 @@
 
 def mimetextpatch(s, subtype='plain', display=False):
     '''Return MIME message suitable for a patch.
-    Charset will be detected as utf-8 or (possibly fake) us-ascii.
+    Charset will be detected by first trying to decode as us-ascii, then utf-8,
+    and finally the global encodings. If all those fail, fall back to
+    ISO-8859-1, an encoding with that allows all byte sequences.
     Transfer encodings will be used if necessary.'''
 
-    cs = 'us-ascii'
+    def codec2iana(encoding):
+        encoding = email.charset.Charset(encoding).input_charset.lower()
+        
+        if encoding.startswith("iso") and not encoding.startswith("iso-"):
+            return "iso-" + encoding[3:]
+        return encoding
+    
+    cs = "iso-8859-1" # a "safe" encoding with no invalid byte sequences
     if not display:
         try:
             s.decode('us-ascii')
+            cs = 'us-ascii'
         except UnicodeDecodeError:
             try:
                 s.decode('utf-8')
                 cs = 'utf-8'
             except UnicodeDecodeError:
-                # We'll go with us-ascii as a fallback.
-                pass
+                try:
+                    s.decode(encoding.encoding)
+                    cs = encoding.encoding
+                except UnicodeDecodeError:
+                    try: 
+                        s.decode(encoding.fallbackencoding)
+                        cs = encoding.fallbackencoding
+                    except UnicodeDecodeError
+                        # fall back to ISO-8859-1
+                        pass
 
-    return mimetextqp(s, subtype, cs)
+    return mimetextqp(s, subtype, codec2iana(cs))
 
 def mimetextqp(body, subtype, charset):
     '''Return MIME message.