lib: separate scheme info from protocol implementation

[bagder]

This allows builds know about all schemes - but only have the protocol
implementations for those actually built-in.

It further allows multiple protocols to reuse the same protocol setup
and functions for both TLS and non-TLS implementations instead of
needing two (or more) structs.

The scheme information is now in 'struct Curl_scheme' and all the
function pointers for each scheme/protocol implementation are in struct
Curl_protocol.

The URL API now always work with all known protocols.

[icing]

Sounds like a good idea.

[Copilot]

Pull request overview

This PR refactors the cURL library's protocol handling architecture by splitting the Curl_handler struct into two separate structs: Curl_scheme (containing scheme metadata) and Curl_protocol (containing protocol implementation function pointers). This separation allows builds to know about all schemes even when specific protocol implementations are disabled, and enables multiple schemes (like HTTP/HTTPS or WS/WSS) to share the same protocol implementation.

Changes:

• Split Curl_handler into Curl_scheme and Curl_protocol structs in urldata.h
• Updated all protocol files to define separate scheme and protocol structs
• Changed references from conn->handler to conn->scheme and handler->field to scheme->field or scheme->run->field throughout the codebase
• Simplified scripts/schemetable.c by removing conditional compilation from the scheme table
• Fixed header guards in vssh files

Reviewed changes

Copilot reviewed 59 out of 59 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
lib/urldata.h	Defines new Curl_protocol and Curl_scheme structs, replacing Curl_handler
lib/url.c	Updates scheme lookup and connection handling to use new structs
lib/multi.c	Updates protocol handler function calls to use conn->scheme->run->*
lib/transfer.c	Updates protocol checks to use conn->scheme->protocol
lib/ws.c	Splits WebSocket handler into protocol and scheme structs
lib/http.c	Splits HTTP/HTTPS handlers, sharing Curl_protocol_http implementation
lib/ftp.c	Splits FTP/FTPS handlers, sharing Curl_protocol_ftp implementation
lib/smtp.c, pop3.c, imap.c	Email protocols split handlers, share protocol implementations
lib/ldap.c, openldap.c	LDAP protocol split across files with shared implementation
lib/vssh/*	SSH protocols updated with scheme/protocol split and header guard fixes
scripts/schemetable.c	Simplified scheme table generation without conditional compilation

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[bagder]

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🤖 Augment PR Summary

Summary: This PR refactors libcurl’s protocol metadata so URL schemes can be known independently of whether the corresponding protocol implementation is compiled in.

Changes:

• Split the former struct Curl_handler into struct Curl_scheme (name/protocol/family/flags/defport) and struct Curl_protocol (function-pointer vtable).
• Update struct connectdata to store conn->scheme/conn->given instead of conn->handler.
• Convert many protocol modules (HTTP, FTP, IMAP/POP3/SMTP, RTSP, TFTP, FILE, DICT, MQTT, SMB, RTMP, WS, LDAP, SSH) to define a protocol vtable plus one-or-more scheme descriptors (e.g., TLS/non-TLS variants reusing the same vtable).
• Adjust core connection/transfer state machines (multi, transfer, connect, cookie, etc.) to read flags/protocol IDs from the scheme and call implementations via scheme->run.
• Rename scheme lookup APIs to Curl_get_scheme()/Curl_getn_scheme() and update URL parsing / URL API call sites accordingly.
• Simplify the generated scheme hash table in url.c to return scheme descriptors.

Technical Notes: This lays groundwork for always recognizing all known schemes (even when implementations are disabled) and for sharing a single protocol implementation across multiple schemes (e.g., TLS/non-TLS variants).

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