mimepost: allocate struct on-demand

[bagder]

This makes the easy handle 432 bytes smaller (totally 5352 bytes on my rather maximized Linux 64 bit build). The 440 byte mimepost struct is now allocated only when needed.

[bagder]

augment review

[augmentcode]

🤖 Augment PR Summary

Summary: Shrinks the per-easy-handle footprint by making the MIME post container optional instead of always embedded.

Changes:

• Replaces embedded curl_mimepart mimepost with curl_mimepart *mimepostp in struct UserDefined.
• Allocates/duplicates/frees the MIME-post part only when needed (setopt/duphandle/cleanup paths).
• Updates HTTP/SMTP/IMAP code paths to use the new pointer when configuring upload readers and preparing MIME headers.
• Adjusts the unit struct-size guard (unit3214) to the new expected Curl_easy size.

Technical Notes: Call sites must now handle mimepostp == NULL as “no MIME post configured”.

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[augmentcode]

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[Copilot]

Pull request overview

This pull request optimizes memory usage in the curl easy handle by converting the mimepost field from an embedded struct to an on-demand allocated pointer. This reduces the size of the easy handle by approximately 432-480 bytes by only allocating the 440-byte mimepost struct when MIME POST functionality is actually used.

Changes:

• Changed mimepost from an embedded curl_mimepart struct to a pointer mimepostp in struct UserDefined
• Updated the IS_MIME_POST macro to check pointer presence instead of the kind field
• Modified initialization, cleanup, duplication, and usage code across multiple files to handle the pointer-based approach

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
lib/urldata.h	Changed mimepost from embedded struct to pointer mimepostp, updated IS_MIME_POST macro
lib/url.c	Removed initialization of embedded struct, updated cleanup to handle pointer
lib/setopt.c	Extracted mime post setup into setopt_mimepost function with on-demand allocation
lib/easy.c	Updated duplication logic to allocate and copy mimepost pointer on-demand
lib/http.c	Updated to use pointer instead of taking address of embedded struct
lib/smtp.c	Updated to use pointer and IS_MIME_POST macro for consistency
lib/imap.c	Updated to use pointer and IS_MIME_POST macro for consistency
tests/unit/unit3214.c	Updated maximum allowed size for CURL_EASY handle (5850 → 5370 bytes)

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