httppeer: extract code for HTTP header spanning

httppeer: extract code for HTTP header spanning

A second consumer of HTTP header spanning will soon be introduced.
Factor out the code to do this so it can be reused.

--- a/mercurial/hgweb/protocol.py	Tue Jan 10 11:20:32 2017 -0800
+++ b/mercurial/hgweb/protocol.py	Sat Dec 24 14:46:02 2016 -0700
@@ -25,6 +25,20 @@
 HGTYPE = 'application/mercurial-0.1'
 HGERRTYPE = 'application/hg-error'
 
+def decodevaluefromheaders(req, headerprefix):
+    """Decode a long value from multiple HTTP request headers."""
+    chunks = []
+    i = 1
+    while True:
+        v = req.env.get('HTTP_%s_%d' % (
+            headerprefix.upper().replace('-', '_'), i))
+        if v is None:
+            break
+        chunks.append(v)
+        i += 1
+
+    return ''.join(chunks)
+
 class webproto(wireproto.abstractserverproto):
     def __init__(self, req, ui):
         self.req = req
@@ -53,15 +67,9 @@
             args.update(cgi.parse_qs(
                 self.req.read(postlen), keep_blank_values=True))
             return args
-        chunks = []
-        i = 1
-        while True:
-            h = self.req.env.get('HTTP_X_HGARG_' + str(i))
-            if h is None:
-                break
-            chunks += [h]
-            i += 1
-        args.update(cgi.parse_qs(''.join(chunks), keep_blank_values=True))
+
+        argvalue = decodevaluefromheaders(self.req, 'X-HgArg')
+        args.update(cgi.parse_qs(argvalue, keep_blank_values=True))
         return args
     def getfile(self, fp):
         length = int(self.req.env['CONTENT_LENGTH'])

--- a/mercurial/httppeer.py	Tue Jan 10 11:20:32 2017 -0800
+++ b/mercurial/httppeer.py	Sat Dec 24 14:46:02 2016 -0700
@@ -53,6 +53,26 @@
     reader.__class__ = readerproxy
     return reader
 
+def encodevalueinheaders(value, header, limit):
+    """Encode a string value into multiple HTTP headers.
+
+    ``value`` will be encoded into 1 or more HTTP headers with the names
+    ``header-<N>`` where ``<N>`` is an integer starting at 1. Each header
+    name + value will be at most ``limit`` bytes long.
+
+    Returns an iterable of 2-tuples consisting of header names and values.
+    """
+    fmt = header + '-%s'
+    valuelen = limit - len(fmt % '000') - len(': \r\n')
+    result = []
+
+    n = 0
+    for i in xrange(0, len(value), valuelen):
+        n += 1
+        result.append((fmt % str(n), value[i:i + valuelen]))
+
+    return result
+
 class httppeer(wireproto.wirepeer):
     def __init__(self, ui, path):
         self.path = path
@@ -135,13 +155,9 @@
             if headersize > 0:
                 # The headers can typically carry more data than the URL.
                 encargs = urlreq.urlencode(sorted(args.items()))
-                headerfmt = 'X-HgArg-%s'
-                contentlen = headersize - len(headerfmt % '000' + ': \r\n')
-                headernum = 0
-                for i in xrange(0, len(encargs), contentlen):
-                    headernum += 1
-                    header = headerfmt % str(headernum)
-                    headers[header] = encargs[i:i + contentlen]
+                for header, value in encodevalueinheaders(encargs, 'X-HgArg',
+                                                          headersize):
+                    headers[header] = value
                     varyheaders.append(header)
             else:
                 q += sorted(args.items())