cmake: optimize building examples in CI

In CI we want to ensure that examples build cleanly, but we don't want
to actually run them there. Meaning it's enough to just compile, but not
link them in CI. Saving time up to 2-4x (MSVC), and disk space up
to 1.2GB (or 8-70x).

Add a new cmake target that compiles all examples without linking them
into runnable binaries. Keep a full build for a single example to test
if it links correctly.

Also:
- CI: switch over all `curl-examples` targets to `curl-examples-build`
- GHA/linux-old: build examples in one of the cmake builds.

Result highlights:

Job                 | Bef. |  Bef. | Aft. | Aft. |
:------------------ | ---: | ----: | ---: |----: |
cygwin              |  15s |   9MB |  10s |  1MB |
msys                |  13s |   8MB |   7s |  1MB |
dl-mingw 15         |  39s |  113M |  34s |  2MB |
dl-mingw 9.5.0      |  49s | 115MB |  42s |  2MB |
dl-mingw 7.3.0      |  19s | 113MB |  14s |  2MB |
dl-mingw 6.4.0      |   9s |  12MB |   7s |  4MB |
Linux cross         |  19s |  28MB |  19s |  2MB |
MSVC UWP            |  65s | 374MB |   9s | 17MB |
MSVC x64            |  22s | 846MB |   9s | 17MB |
VS2010              |  48s | 105MB |  15s |  9MB |
VS2022 clang-cl     | 195s | 1.2GB |  51s | 20MB |
iOS Xcode           |   8s |       |   5s |      |
macOS LibreSSL      |  16s |       |  11s |      |
Linux aws-lc        |   3s |       |   1s |      |

Follow-up to dda251ef10 #18232

Closes #18209

.github/workflows/http3-linux.yml

@@ -546,7 +546,7 @@ jobs:
       - name: 'build examples'
         run: |
           if [ "${MATRIX_BUILD}" = 'cmake' ]; then
-            cmake --build bld --verbose --target curl-examples
+            cmake --build bld --verbose --target curl-examples-build
           else
             make -C bld V=1 examples
           fi

.github/workflows/linux-old.yml

@@ -123,6 +123,9 @@ jobs:
       - name: 'cmake run tests'
         run: make -C bld-cares test-ci
 
+      - name: 'cmake build examples'
+        run: make -C bld-cares curl-examples-build
+
       - name: 'autoreconf'
         run: autoreconf -if
 

.github/workflows/linux.yml

@@ -698,7 +698,7 @@ jobs:
         if: ${{ matrix.build.make-custom-target != 'tidy' }}
         run: |
           if [ "${MATRIX_BUILD}" = 'cmake' ]; then
-            ${MATRIX_MAKE_PREFIX} cmake --build bld --verbose --target curl-examples
+            ${MATRIX_MAKE_PREFIX} cmake --build bld --verbose --target curl-examples-build
           else
             ${MATRIX_MAKE_PREFIX} make -C bld V=1 examples
           fi

.github/workflows/macos.yml

@@ -199,7 +199,7 @@ jobs:
       - name: 'build examples'
         run: |
           if [ "${MATRIX_BUILD}" = 'cmake' ]; then
-            cmake --build bld ${MATRIX_OPTIONS} --parallel 4 --target curl-examples --verbose
+            cmake --build bld ${MATRIX_OPTIONS} --parallel 4 --target curl-examples-build --verbose
           else
             make -C bld examples V=1
           fi
@@ -520,7 +520,7 @@ jobs:
         if: ${{ contains(matrix.build.name, '+examples') }}
         run: |
           if [ "${MATRIX_BUILD}" = 'cmake' ]; then
-            cmake --build bld --verbose --target curl-examples
+            cmake --build bld --verbose --target curl-examples-build
           else
             make -C bld examples V=1
           fi

.github/workflows/non-native.yml

@@ -83,7 +83,7 @@ jobs:
               time cmake --build bld --target test-ci
             fi
             echo '::group::build examples'
-            time cmake --build bld --target curl-examples
+            time cmake --build bld --target curl-examples-build
             echo '::endgroup::'
 
   openbsd:
@@ -128,7 +128,7 @@ jobs:
               time cmake --build bld --target test-ci
             fi
             echo '::group::build examples'
-            time cmake --build bld --target curl-examples
+            time cmake --build bld --target curl-examples-build
             echo '::endgroup::'
 
   freebsd:
@@ -232,7 +232,7 @@ jobs:
             if [ "${MATRIX_DESC#*!examples*}" = "${MATRIX_DESC}" ]; then
               echo '::group::build examples'
               if [ "${MATRIX_BUILD}" = 'cmake' ]; then
-                time cmake --build bld --target curl-examples
+                time cmake --build bld --target curl-examples-build
               else
                 time make -C bld examples
               fi
@@ -361,7 +361,7 @@ jobs:
       - name: 'build examples'
         run: |
           if [ "${MATRIX_BUILD}" = 'cmake' ]; then
-            cmake --build bld --target curl-examples
+            cmake --build bld --target curl-examples-build
           else
             make -C bld examples
           fi
@@ -473,7 +473,7 @@ jobs:
         if: ${{ matrix.build == 'cmake' }}  # skip for autotools to save time
         run: |
           if [ "${MATRIX_BUILD}" = 'cmake' ]; then
-            cmake --build bld --target curl-examples
+            cmake --build bld --target curl-examples-build
           else
             make -C bld examples
           fi

.github/workflows/windows.yml

@@ -177,7 +177,7 @@ jobs:
         run: |
           PATH=/usr/bin
           if [ "${MATRIX_BUILD}" = 'cmake' ]; then
-            cmake --build bld --verbose --target curl-examples
+            cmake --build bld --verbose --target curl-examples-build
           else
             make -C bld V=1 examples
           fi
@@ -404,7 +404,7 @@ jobs:
         timeout-minutes: 5
         run: |
           if [ "${MATRIX_BUILD}" = 'cmake' ]; then
-            cmake --build bld --verbose --target curl-examples
+            cmake --build bld --verbose --target curl-examples-build
           else
             make -C bld V=1 examples
           fi
@@ -589,7 +589,7 @@ jobs:
         timeout-minutes: 5
         run: |
           PATH="/d/my-cache/${MATRIX_DIR}/bin:$PATH"
-          cmake --build bld --target curl-examples
+          cmake --build bld --target curl-examples-build
 
       - name: 'disk space used'
         run: du -sh .; echo; du -sh -t 250KB ./*; echo; du -h -t 250KB bld
@@ -687,7 +687,7 @@ jobs:
         if: ${{ matrix.compiler != 'clang-tidy' }}  # Save time by skipping this for clang-tidy
         run: |
           if [ "${MATRIX_BUILD}" = 'cmake' ]; then
-            cmake --build bld --target curl-examples
+            cmake --build bld --target curl-examples-build
           else
             make -C bld examples
           fi
@@ -935,7 +935,7 @@ jobs:
       - name: 'build examples'
         timeout-minutes: 5
         if: ${{ contains(matrix.name, '+examples') }}
-        run: cmake --build bld --config "${MATRIX_TYPE}" --parallel 5 --target curl-examples
+        run: cmake --build bld --config "${MATRIX_TYPE}" --parallel 5 --target curl-examples-build
 
       - name: 'disk space used'
         run: du -sh .; echo; du -sh -t 250KB ./*; echo; du -h -t 250KB bld

appveyor.sh

@@ -163,7 +163,7 @@ fi
 
 if [ "${EXAMPLES}" = 'ON' ] && \
    [ "${BUILD_SYSTEM}" = 'CMake' ]; then
-  time cmake --build _bld --config "${PRJ_CFG}" --parallel 2 --target curl-examples
+  time cmake --build _bld --config "${PRJ_CFG}" --parallel 2 --target curl-examples-build
 fi
 
 # disk space used

docs/examples/CMakeLists.txt

@@ -28,10 +28,40 @@ add_custom_target(curl-examples)
 curl_transform_makefile_inc("Makefile.inc" "${CMAKE_CURRENT_BINARY_DIR}/Makefile.inc.cmake")
 include("${CMAKE_CURRENT_BINARY_DIR}/Makefile.inc.cmake")
 
-foreach(_target IN LISTS check_PROGRAMS)
+set(_all_canary "")
+set(_all "all")
+foreach(_target IN LISTS check_PROGRAMS _all)  # keep '_all' last
   set(_target_name "curl-example-${_target}")
-  add_executable(${_target_name} EXCLUDE_FROM_ALL "${_target}.c")
-  add_dependencies(curl-examples ${_target_name})
+  if(_target STREQUAL "all")
+    if(CMAKE_VERSION VERSION_GREATER_EQUAL 3.12)
+      set(_examples_c "${check_PROGRAMS}")
+      list(TRANSFORM _examples_c APPEND ".c")
+      add_library(${_target_name} OBJECT EXCLUDE_FROM_ALL ${_examples_c})
+      if(MSVC AND NOT CMAKE_C_COMPILER_ID STREQUAL "Clang")
+        # CMake generates a static library for the OBJECT target. Silence these 'lib.exe' warnings:
+        # warning LNK4006: main already defined in ....obj; second definition ignored
+        # warning LNK4221: This object file does not define any previously undefined public symbols,
+        #                  so it will not be used by any link operation that consumes this library
+        if(CMAKE_VERSION VERSION_GREATER_EQUAL 3.13)
+          set_target_properties(${_target_name} PROPERTIES STATIC_LIBRARY_OPTIONS "-ignore:4006;-ignore:4221")
+        else()
+          set_target_properties(${_target_name} PROPERTIES STATIC_LIBRARY_FLAGS "-ignore:4006 -ignore:4221")
+        endif()
+      endif()
+    else()
+      set(_examples_c "")
+      foreach(_src IN LISTS check_PROGRAMS)
+        list(APPEND _examples_c "${_src}.c")
+      endforeach()
+      add_library(${_target_name} STATIC EXCLUDE_FROM_ALL ${_examples_c})
+    endif()
+    add_custom_target(curl-examples-build)  # Special target to compile all tests quickly and build a single test to probe linkage
+    add_dependencies(curl-examples-build ${_target_name} ${_all_canary})  # Include a full build of a single test
+  else()
+    set(_all_canary ${_target_name})  # Save the last test for the curl-examples-build target
+    add_executable(${_target_name} EXCLUDE_FROM_ALL "${_target}.c")
+    add_dependencies(curl-examples ${_target_name})
+  endif()
   target_link_libraries(${_target_name} ${LIB_SELECTED} ${CURL_NETWORK_AND_TIME_LIBS})
   target_compile_definitions(${_target_name} PRIVATE "CURL_NO_OLDIES"
     "$<$<BOOL:${WIN32}>:WIN32_LEAN_AND_MEAN>" "$<$<BOOL:${MSVC}>:_CRT_SECURE_NO_DEPRECATE>")