content_encoding: brotli and others, pass through 0-length writes

- curl's transfer handling may write 0-length chunks at the end of the
  download with an EOS flag. (HTTP/2 does this commonly)

- content encoders need to pass-through such a write and not count this
  as error in case they are finished decoding

Fixes #13209
Fixes #13212
Closes #13219

tests/http/testenv/env.py

@@ -129,10 +129,11 @@ class EnvConfig:
         self.htdocs_dir = os.path.join(self.gen_dir, 'htdocs')
         self.tld = 'http.curl.se'
         self.domain1 = f"one.{self.tld}"
+        self.domain1brotli = f"brotli.one.{self.tld}"
         self.domain2 = f"two.{self.tld}"
         self.proxy_domain = f"proxy.{self.tld}"
         self.cert_specs = [
-            CertificateSpec(domains=[self.domain1, 'localhost'], key_type='rsa2048'),
+            CertificateSpec(domains=[self.domain1, self.domain1brotli, 'localhost'], key_type='rsa2048'),
             CertificateSpec(domains=[self.domain2], key_type='rsa2048'),
             CertificateSpec(domains=[self.proxy_domain, '127.0.0.1'], key_type='rsa2048'),
             CertificateSpec(name="clientsX", sub_specs=[
@@ -376,6 +377,10 @@ class Env:
     def domain1(self) -> str:
         return self.CONFIG.domain1
 
+    @property
+    def domain1brotli(self) -> str:
+        return self.CONFIG.domain1brotli
+
     @property
     def domain2(self) -> str:
         return self.CONFIG.domain2

tests/http/testenv/httpd.py

@@ -50,6 +50,7 @@ class Httpd:
         'alias', 'env', 'filter', 'headers', 'mime', 'setenvif',
         'socache_shmcb',
         'rewrite', 'http2', 'ssl', 'proxy', 'proxy_http', 'proxy_connect',
+        'brotli',
         'mpm_event',
     ]
     COMMON_MODULES_DIRS = [
@@ -203,6 +204,7 @@ class Httpd:
 
     def _write_config(self):
         domain1 = self.env.domain1
+        domain1brotli = self.env.domain1brotli
         creds1 = self.env.get_credentials(domain1)
         domain2 = self.env.domain2
         creds2 = self.env.get_credentials(domain2)
@@ -285,6 +287,24 @@ class Httpd:
                 f'</VirtualHost>',
                 f'',
             ])
+            # Alternate to domain1 with BROTLI compression
+            conf.extend([  # https host for domain1, h1 + h2
+                f'<VirtualHost *:{self.env.https_port}>',
+                f'    ServerName {domain1brotli}',
+                f'    Protocols h2 http/1.1',
+                f'    SSLEngine on',
+                f'    SSLCertificateFile {creds1.cert_file}',
+                f'    SSLCertificateKeyFile {creds1.pkey_file}',
+                f'    DocumentRoot "{self._docs_dir}"',
+                f'    SetOutputFilter BROTLI_COMPRESS',
+            ])
+            conf.extend(self._curltest_conf(domain1))
+            if domain1 in self._extra_configs:
+                conf.extend(self._extra_configs[domain1])
+            conf.extend([
+                f'</VirtualHost>',
+                f'',
+            ])
             conf.extend([  # https host for domain2, no h2
                 f'<VirtualHost *:{self.env.https_port}>',
                 f'    ServerName {domain2}',