Fix IndexDefect errors in httpclient on invalid/weird headers (#22886)

Continuation of https://github.com/nim-lang/Nim/pull/19262

Fixes https://github.com/nim-lang/Nim/issues/19261

The parsing code is still too lenient (e.g. it will happily parse header
names with spaces in them, which is outright invalid by the spec), but I
didn't want to touch it beyond the simple changes to make sure that
`std/httpclient` won't throw `IndexDefect`s like it does now on those
cases:
- Multiline header values
- No colon after the header name
- No value after the header name + colon

One question remains - should I keep `toCaseInsensitive` exported in
`httpcore` or just copy-paste the implementation?

---------

Co-authored-by: Andreas Rumpf <rumpf_a@web.de>

lib/pure/httpclient.nim

@@ -856,6 +856,7 @@ proc parseResponse(client: HttpClient | AsyncHttpClient,
   var parsedStatus = false
   var linei = 0
   var fullyRead = false
+  var lastHeaderName = ""
   var line = ""
   result.headers = newHttpHeaders()
   while true:
@@ -890,16 +891,29 @@ proc parseResponse(client: HttpClient | AsyncHttpClient,
       parsedStatus = true
     else:
       # Parse headers
-      var name = ""
-      var le = parseUntil(line, name, ':', linei)
-      if le <= 0: httpError("invalid headers")
-      inc(linei, le)
-      if line[linei] != ':': httpError("invalid headers")
-      inc(linei) # Skip :
-
-      result.headers.add(name, line[linei .. ^1].strip())
-      if result.headers.len > headerLimit:
-        httpError("too many headers")
+      # There's at least one char because empty lines are handled above (with client.close)
+      if line[0] in {' ', '\t'}:
+        # Check if it's a multiline header value, if so, append to the header we're currently parsing
+        # This works because a line with a header must start with the header name without any leading space
+        # See https://datatracker.ietf.org/doc/html/rfc7230, section 3.2 and 3.2.4
+        # Multiline headers are deprecated in the spec, but it's better to parse them than crash
+        if lastHeaderName == "":
+          # Some extra unparsable lines in the HTTP output - we ignore them
+          discard
+        else:
+          result.headers.table[result.headers.toCaseInsensitive(lastHeaderName)][^1].add "\n" & line
+      else:
+        var name = ""
+        var le = parseUntil(line, name, ':', linei)
+        if le <= 0: httpError("Invalid headers - received empty header name")
+        if line.len == le: httpError("Invalid headers - no colon after header name")
+        inc(linei, le) # Skip the parsed header name
+        inc(linei) # Skip :
+        # If we want to be HTTP spec compliant later, error on linei == line.len (for empty header value)
+        lastHeaderName = name # Remember the header name for the possible multi-line header
+        result.headers.add(name, line[linei .. ^1].strip())
+        if result.headers.len > headerLimit:
+          httpError("too many headers")
 
   if not fullyRead:
     httpError("Connection was closed before full request has been made")

lib/pure/httpcore.nim

@@ -126,7 +126,8 @@ func toTitleCase(s: string): string =
     result[i] = if upper: toUpperAscii(s[i]) else: toLowerAscii(s[i])
     upper = s[i] == '-'
 
-func toCaseInsensitive(headers: HttpHeaders, s: string): string {.inline.} =
+func toCaseInsensitive*(headers: HttpHeaders, s: string): string {.inline.} =
+  ## For internal usage only. Do not use.
   return if headers.isTitleCase: toTitleCase(s) else: toLowerAscii(s)
 
 func newHttpHeaders*(titleCase=false): HttpHeaders =