feat(headers): dynamically create cononicalized headers

Instead of a static list dynamically add entries in proper casing.
Also allowing to add non-standard ones through `setcanonic`.

closes #442
fixes #440

4 files changed, 206 insertions, 97 deletions

diff --git a/.github/workflows/build.yml b/.github/workflows/build.yml
index 53d6ebc..b8043c5 100644
--- a/.github/workflows/build.yml
+++ b/.github/workflows/build.yml
@@ -48,5 +48,6 @@ jobs:
           lua ltn12test.lua
           lua mimetest.lua
           lua urltest.lua
+          lua headerstest.lua
           lua test_socket_error.lua
           kill %1
diff --git a/docs/socket.html b/docs/socket.html
index 983f1c9..570f6d5 100644
--- a/docs/socket.html
+++ b/docs/socket.html
@@ -65,13 +65,33 @@ whenever the field name is sent out.
 </p>
 
 <p>
-You can obtain the <tt>headers</tt> namespace if case run-time
-modifications are required by running:
+If a lowercase field name is not present as a key,
+<tt>socket.headers.canonic</tt> automatically derives a
+capitalization by uppercasing the first letter of each
+hyphen-separated word (e.g. <tt>x-request-id</tt> becomes
+<tt>X-Request-Id</tt>), and caches the result for later lookups.
+A small set of headers whose canonic capitalization is
+irregular (such as <tt>ETag</tt>, <tt>Content-MD5</tt>,
+<tt>MIME-Version</tt> and <tt>WWW-Authenticate</tt>) are kept as
+explicit entries in the table and are not affected by this rule.
+Because of this, <tt>pairs(socket.headers.canonic)</tt> only
+shows the irregular entries plus any field name already looked
+up or added &mdash; it is not an exhaustive list of every header
+name the library knows how to capitalize.
+</p>
+
+<p>
+You can register a custom canonic capitalization &mdash; for a
+header the automatic rule would not reproduce, such as a custom
+all-caps acronym &mdash; with <tt>socket.headers.setcanonic</tt>:
 </p>
 
 <pre class="example">
 -- loads the headers module
-local headers = require("headers")
+local headers = require("socket.headers")
+
+-- register a custom capitalization
+headers.setcanonic("X-Request-ID")
 </pre>
 
 
diff --git a/src/headers.lua b/src/headers.lua
index 1eb8223..f8b8ef8 100644
--- a/src/headers.lua
+++ b/src/headers.lua
@@ -7,98 +7,55 @@ local socket = require("socket")
 socket.headers = {}
 local _M = socket.headers
 
-_M.canonic = {
-    ["accept"] = "Accept",
-    ["accept-charset"] = "Accept-Charset",
-    ["accept-encoding"] = "Accept-Encoding",
-    ["accept-language"] = "Accept-Language",
-    ["accept-ranges"] = "Accept-Ranges",
-    ["action"] = "Action",
-    ["alternate-recipient"] = "Alternate-Recipient",
-    ["age"] = "Age",
-    ["allow"] = "Allow",
-    ["arrival-date"] = "Arrival-Date",
-    ["authorization"] = "Authorization",
-    ["bcc"] = "Bcc",
-    ["cache-control"] = "Cache-Control",
-    ["cc"] = "Cc",
-    ["comments"] = "Comments",
-    ["connection"] = "Connection",
-    ["content-description"] = "Content-Description",
-    ["content-disposition"] = "Content-Disposition",
-    ["content-encoding"] = "Content-Encoding",
-    ["content-id"] = "Content-ID",
-    ["content-language"] = "Content-Language",
-    ["content-length"] = "Content-Length",
-    ["content-location"] = "Content-Location",
-    ["content-md5"] = "Content-MD5",
-    ["content-range"] = "Content-Range",
-    ["content-transfer-encoding"] = "Content-Transfer-Encoding",
-    ["content-type"] = "Content-Type",
-    ["cookie"] = "Cookie",
-    ["date"] = "Date",
-    ["diagnostic-code"] = "Diagnostic-Code",
-    ["dsn-gateway"] = "DSN-Gateway",
-    ["etag"] = "ETag",
-    ["expect"] = "Expect",
-    ["expires"] = "Expires",
-    ["final-log-id"] = "Final-Log-ID",
-    ["final-recipient"] = "Final-Recipient",
-    ["from"] = "From",
-    ["host"] = "Host",
-    ["if-match"] = "If-Match",
-    ["if-modified-since"] = "If-Modified-Since",
-    ["if-none-match"] = "If-None-Match",
-    ["if-range"] = "If-Range",
-    ["if-unmodified-since"] = "If-Unmodified-Since",
-    ["in-reply-to"] = "In-Reply-To",
-    ["keywords"] = "Keywords",
-    ["last-attempt-date"] = "Last-Attempt-Date",
-    ["last-modified"] = "Last-Modified",
-    ["location"] = "Location",
-    ["max-forwards"] = "Max-Forwards",
-    ["message-id"] = "Message-ID",
-    ["mime-version"] = "MIME-Version",
-    ["original-envelope-id"] = "Original-Envelope-ID",
-    ["original-recipient"] = "Original-Recipient",
-    ["pragma"] = "Pragma",
-    ["proxy-authenticate"] = "Proxy-Authenticate",
-    ["proxy-authorization"] = "Proxy-Authorization",
-    ["range"] = "Range",
-    ["received"] = "Received",
-    ["received-from-mta"] = "Received-From-MTA",
-    ["references"] = "References",
-    ["referer"] = "Referer",
-    ["remote-mta"] = "Remote-MTA",
-    ["reply-to"] = "Reply-To",
-    ["reporting-mta"] = "Reporting-MTA",
-    ["resent-bcc"] = "Resent-Bcc",
-    ["resent-cc"] = "Resent-Cc",
-    ["resent-date"] = "Resent-Date",
-    ["resent-from"] = "Resent-From",
-    ["resent-message-id"] = "Resent-Message-ID",
-    ["resent-reply-to"] = "Resent-Reply-To",
-    ["resent-sender"] = "Resent-Sender",
-    ["resent-to"] = "Resent-To",
-    ["retry-after"] = "Retry-After",
-    ["return-path"] = "Return-Path",
-    ["sender"] = "Sender",
-    ["server"] = "Server",
-    ["smtp-remote-recipient"] = "SMTP-Remote-Recipient",
-    ["status"] = "Status",
-    ["subject"] = "Subject",
-    ["te"] = "TE",
-    ["to"] = "To",
-    ["trailer"] = "Trailer",
-    ["transfer-encoding"] = "Transfer-Encoding",
-    ["upgrade"] = "Upgrade",
-    ["user-agent"] = "User-Agent",
-    ["vary"] = "Vary",
-    ["via"] = "Via",
-    ["warning"] = "Warning",
-    ["will-retry-until"] = "Will-Retry-Until",
-    ["www-authenticate"] = "WWW-Authenticate",
-    ["x-mailer"] = "X-Mailer",
-}
+-- capitalizes the first letter of each hyphen-separated word, lowercases
+-- the rest (e.g. "x-request-id" -> "X-Request-Id")
+local function titlecase(header)
+    return (header:gsub("(%a)([%w]*)", function(a, b) return a:upper()..b:lower() end))
+end
 
-return _M
\ No newline at end of file
+_M.canonic = {}
+
+setmetatable(_M.canonic, {
+    __index = function(t, key)
+        if type(key) ~= "string" then
+            return nil
+        end
+
+        local lower = key:lower()
+        local v = rawget(t, lower)
+        if v then
+            return v
+        end
+
+        v = titlecase(lower)
+        rawset(t, lower, v)
+        return v
+    end
+})
+
+-- adds a header with a given canonical capitalization, e.g. for headers
+-- whose capitalization titlecase(header) would not reproduce correctly
+function _M.setcanonic(header)
+    _M.canonic[header:lower()] = header
+end
+
+-- headers whose canonical capitalization titlecase() does not reproduce
+-- (acronyms and other irregular capitalization). Anything not listed here
+-- is generated and cached on first lookup by the __index above.
+_M.setcanonic("Content-ID")
+_M.setcanonic("Content-MD5")
+_M.setcanonic("DSN-Gateway")
+_M.setcanonic("ETag")
+_M.setcanonic("Final-Log-ID")
+_M.setcanonic("Message-ID")
+_M.setcanonic("MIME-Version")
+_M.setcanonic("Original-Envelope-ID")
+_M.setcanonic("Received-From-MTA")
+_M.setcanonic("Remote-MTA")
+_M.setcanonic("Reporting-MTA")
+_M.setcanonic("Resent-Message-ID")
+_M.setcanonic("SMTP-Remote-Recipient")
+_M.setcanonic("TE")
+_M.setcanonic("WWW-Authenticate")
+
+return _M
diff --git a/test/headerstest.lua b/test/headerstest.lua
new file mode 100644
index 0000000..1387fc7
--- /dev/null
+++ b/test/headerstest.lua
@@ -0,0 +1,131 @@
+local socket = require("socket")
+local headers = require("socket.headers")
+
+-- the original hardcoded table this file used to contain, kept here so the
+-- refactor to dynamic generation + a small anomaly table can be checked for
+-- regressions.
+local canonic = {
+    ["accept"] = "Accept",
+    ["accept-charset"] = "Accept-Charset",
+    ["accept-encoding"] = "Accept-Encoding",
+    ["accept-language"] = "Accept-Language",
+    ["accept-ranges"] = "Accept-Ranges",
+    ["action"] = "Action",
+    ["alternate-recipient"] = "Alternate-Recipient",
+    ["age"] = "Age",
+    ["allow"] = "Allow",
+    ["arrival-date"] = "Arrival-Date",
+    ["authorization"] = "Authorization",
+    ["bcc"] = "Bcc",
+    ["cache-control"] = "Cache-Control",
+    ["cc"] = "Cc",
+    ["comments"] = "Comments",
+    ["connection"] = "Connection",
+    ["content-description"] = "Content-Description",
+    ["content-disposition"] = "Content-Disposition",
+    ["content-encoding"] = "Content-Encoding",
+    ["content-id"] = "Content-ID",
+    ["content-language"] = "Content-Language",
+    ["content-length"] = "Content-Length",
+    ["content-location"] = "Content-Location",
+    ["content-md5"] = "Content-MD5",
+    ["content-range"] = "Content-Range",
+    ["content-transfer-encoding"] = "Content-Transfer-Encoding",
+    ["content-type"] = "Content-Type",
+    ["cookie"] = "Cookie",
+    ["date"] = "Date",
+    ["diagnostic-code"] = "Diagnostic-Code",
+    ["dsn-gateway"] = "DSN-Gateway",
+    ["etag"] = "ETag",
+    ["expect"] = "Expect",
+    ["expires"] = "Expires",
+    ["final-log-id"] = "Final-Log-ID",
+    ["final-recipient"] = "Final-Recipient",
+    ["from"] = "From",
+    ["host"] = "Host",
+    ["if-match"] = "If-Match",
+    ["if-modified-since"] = "If-Modified-Since",
+    ["if-none-match"] = "If-None-Match",
+    ["if-range"] = "If-Range",
+    ["if-unmodified-since"] = "If-Unmodified-Since",
+    ["in-reply-to"] = "In-Reply-To",
+    ["keywords"] = "Keywords",
+    ["last-attempt-date"] = "Last-Attempt-Date",
+    ["last-modified"] = "Last-Modified",
+    ["location"] = "Location",
+    ["max-forwards"] = "Max-Forwards",
+    ["message-id"] = "Message-ID",
+    ["mime-version"] = "MIME-Version",
+    ["original-envelope-id"] = "Original-Envelope-ID",
+    ["original-recipient"] = "Original-Recipient",
+    ["pragma"] = "Pragma",
+    ["proxy-authenticate"] = "Proxy-Authenticate",
+    ["proxy-authorization"] = "Proxy-Authorization",
+    ["range"] = "Range",
+    ["received"] = "Received",
+    ["received-from-mta"] = "Received-From-MTA",
+    ["references"] = "References",
+    ["referer"] = "Referer",
+    ["remote-mta"] = "Remote-MTA",
+    ["reply-to"] = "Reply-To",
+    ["reporting-mta"] = "Reporting-MTA",
+    ["resent-bcc"] = "Resent-Bcc",
+    ["resent-cc"] = "Resent-Cc",
+    ["resent-date"] = "Resent-Date",
+    ["resent-from"] = "Resent-From",
+    ["resent-message-id"] = "Resent-Message-ID",
+    ["resent-reply-to"] = "Resent-Reply-To",
+    ["resent-sender"] = "Resent-Sender",
+    ["resent-to"] = "Resent-To",
+    ["retry-after"] = "Retry-After",
+    ["return-path"] = "Return-Path",
+    ["sender"] = "Sender",
+    ["server"] = "Server",
+    ["smtp-remote-recipient"] = "SMTP-Remote-Recipient",
+    ["status"] = "Status",
+    ["subject"] = "Subject",
+    ["te"] = "TE",
+    ["to"] = "To",
+    ["trailer"] = "Trailer",
+    ["transfer-encoding"] = "Transfer-Encoding",
+    ["upgrade"] = "Upgrade",
+    ["user-agent"] = "User-Agent",
+    ["vary"] = "Vary",
+    ["via"] = "Via",
+    ["warning"] = "Warning",
+    ["will-retry-until"] = "Will-Retry-Until",
+    ["www-authenticate"] = "WWW-Authenticate",
+    ["x-mailer"] = "X-Mailer",
+}
+
+local check_canonic = function(lower, expected)
+    local got = headers.canonic[lower]
+    if got ~= expected then
+        print("canonic[" .. lower .. "] = " .. tostring(got) ..
+            ", expected " .. expected)
+        os.exit()
+    end
+end
+
+print("testing all known header fields resolve to their canonical form")
+for lower, expected in pairs(canonic) do
+    check_canonic(lower, expected)
+end
+
+print("testing mixed/upper-case input normalizes correctly")
+check_canonic("Content-Type", "Content-Type")
+check_canonic("CONTENT-TYPE", "Content-Type")
+check_canonic("ETAG", "ETag")
+check_canonic("Www-Authenticate", "WWW-Authenticate")
+
+print("testing novel unregistered headers are auto-titlecased")
+check_canonic("x-request-id", "X-Request-Id")
+check_canonic("x-custom-header-name", "X-Custom-Header-Name")
+check_canonic("single", "Single")
+
+print("testing setcanonic() registers a custom canonical capitalization")
+headers.setcanonic("X-Request-ID")
+check_canonic("x-request-id", "X-Request-ID")
+check_canonic("X-REQUEST-ID", "X-Request-ID")
+
+print("the library passed all tests")