Change #286442

Comments

cmake: fix preconditions for option `CURL_STATIC_CRT=ON`
Limit this option to static-only curl builds.

Enabling the static CRT for other configurations may successfuly build
them, but executing them do run into nasty problems due to using two CRT
copies at the same time. Also note that the static CRT setting applies
to tests too, which revealed these issues.

Also:
- as it turns out, static CRT may be enabled for non-UCRT builds. After
  this patch, such configuration is possible again.
- note, if these preconditions prevent doing a desired static-CRT build
  variation (e.g. a libcurl.dll built against static CRT). You can
  pass the necessary compiler/linker options via standard CMake options/envs.

I discovered these issues while enabling test runs on AppVeyor CI with
all supported MSVC version in various configurations, and
`-DCURL_STATIC_CRT=ON` set.

Symptoms of double CRT:
- `curl -V` output with newline missing between 'Protocols:' and
  'Features:' lines, then runtests unable to detect feature and emitting
  a Perl warning for uninitialized `$feat` variable. (due to parallel
  output buffering for `puts()` and `curl_mprintf()`)
  This is visible in master builds, e.g.:
  CM VS2019, Debug, x64, OpenSSL 3.0 + Schannel, Shared, !verbose, Build-tests:
  ```

  WARNING: this libcurl is Debug-enabled, do not use in production

  curl 8.23.0-DEV (Windows) libcurl/8.23.0-DEV OpenSSL/3.0.18 (Schannel) WinLDAP
  Release-Date: [unreleased]
  Protocols: dict file ftp ftps gopher gophers http https imap imaps ipfs ipns ldap ldaps mqtt mqtts pop3 pop3s rtsp smtp smtps telnet tftp ws wssFeatures: alt-svc AsynchDNS Debug HSTS HTTPS-proxy HTTPSIG IPv6 Kerberos Largefile MultiSSL SPNEGO SSL SSPI threadsafe UnixSockets
  ```
  Ref: https://ci.appveyor.com/project/curlorg/curl/builds/54865570/job/urr2rgnr32hwllw2#L246
  Notice the missing newline before `Features:`, and appearing above
  `WARNING:` instead.
- test1622 failing.
- 'Debug Assertion Failed! Expression: _acrt_first_block == header'
  crashed in both the app and the libcurl.dll it loaded. (possibly)

Follow-up to 61407464bf049ec013879e236e7f0bfae343ae16 #16522
Follow-up to 049352dd80e1ab73945c49a3bac2e4a08db32f98 #16516
Follow-up to edfa537100d6b5d2ac1b829c66757880afe59407 #16456 #16394
Follow-up to 4fc6ebe18a607764194ee23e5aa898a027fe3c60 #1621
Cherry-picked from #23306

Closes #23336

Changed files

• CMakeLists.txt
• docs/INSTALL-CMAKE.md