mime: fix reader stall on small read lengths

The base64 mime encoder stalls when it cannot encode a full 3 byte input
set into the read buffer. The workaround for this limitation was
incomplete and could lead to stalled transfers when the last chunk to
upload was smaller than 4 bytes.

Use a tmp buffer on small reads to allow mime encoders more space to put
their things.

Add test case reproducing the issue and fix.

Reported-by: Alexis Savin
Fixes #15688
Closes #15691

tests/http/test_07_upload.py

@@ -669,6 +669,25 @@ class TestUpload:
         ])
         r.check_stats(count=1, http_status=200, exitcode=0)
 
+    # issue #15688 when posting a form and cr_mime_read() is called with
+    # length < 4, we did not progress
+    @pytest.mark.parametrize("proto", ['http/1.1'])
+    def test_07_62_upload_issue_15688(self, env: Env, httpd, proto):
+        # this length leads to (including multipart formatting) to a
+        # client reader invocation with length 1.
+        upload_len = 196169
+        fname = f'data-{upload_len}'
+        env.make_data_file(indir=env.gen_dir, fname=fname, fsize=upload_len)
+        fdata = os.path.join(env.gen_dir, fname)
+        curl = CurlClient(env=env)
+        url = f'https://{env.authority_for(env.domain1, proto)}/curltest/echo?id=[0-0]'
+        r = curl.http_form(urls=[url], form={
+            'file': f'@{fdata}',
+        }, alpn_proto=proto, extra_args=[
+            '--max-time', '10'
+        ])
+        r.check_stats(count=1, http_status=200, exitcode=0)
+
     # nghttpx is the only server we have that supports TLS early data and
     # has a limit of 16k it announces
     @pytest.mark.skipif(condition=not Env.have_nghttpx(), reason="no nghttpx")