FetchContent: Invoke steps directly and avoid a separate sub-build

The cost of setting up and executing a separate sub-build to do the
download, update and patch steps required for FetchContent population
can be significant with some platforms and CMake generators. Avoid the
sub-build altogether by invoking the step scripts directly.

Previously, if no generator was set (e.g. population was being done in
script mode), a generator needed to be available on the default PATH.
Since we no longer use a sub-build, this restriction is also now gone.

Fixes: #21703

10 files changed, 296 insertions, 103 deletions

diff --git a/Modules/ExternalProject/captured_process_setup.cmake b/Modules/ExternalProject/captured_process_setup.cmake
new file mode 100644
index 0000000..9c8abb1
--- /dev/null
+++ b/Modules/ExternalProject/captured_process_setup.cmake
@@ -0,0 +1,55 @@
+# Distributed under the OSI-approved BSD 3-Clause License.  See accompanying
+# file Copyright.txt or https://cmake.org/licensing for details.
+
+if(quiet)
+  set(capture_output
+    OUTPUT_VARIABLE out_var
+    ERROR_VARIABLE  out_var
+    OUTPUT_STRIP_TRAILING_WHITESPACE
+    ERROR_STRIP_TRAILING_WHITESPACE
+  )
+  set(capture_error_only
+    ERROR_VARIABLE  out_var
+    ERROR_STRIP_TRAILING_WHITESPACE
+  )
+else()
+  unset(capture_output)
+  unset(capture_error_only)
+endif()
+
+set(out_var "")
+set(accumulated_output "")
+
+macro(_ep_message_quiet_capture mode)
+  if("${mode}" STREQUAL "FATAL_ERROR")
+    string(JOIN "" detail "${ARGN}")
+    if(NOT detail STREQUAL "" AND NOT accumulated_output STREQUAL "")
+      string(PREPEND detail "\n")
+    endif()
+    message(FATAL_ERROR "${accumulated_output}${detail}")
+  endif()
+
+  if(quiet)
+    if("${mode}" MATCHES "WARNING")
+      # We can't provide the full CMake backtrace, but we can at least record
+      # the warning message with a sensible prefix
+      string(APPEND accumulated_output "${mode}: ")
+    endif()
+    string(APPEND accumulated_output "${ARGN}\n")
+  else()
+    message(${mode} ${ARGN})
+  endif()
+endmacro()
+
+macro(_ep_accumulate_captured_output)
+  if(NOT "${out_var}" STREQUAL "")
+    string(APPEND accumulated_output "${out_var}\n")
+  endif()
+endmacro()
+
+macro(_ep_command_check_result result)
+  _ep_accumulate_captured_output()
+  if(result)
+    _ep_message_quiet_capture(FATAL_ERROR ${ARGN})
+  endif()
+endmacro()
diff --git a/Modules/ExternalProject/customcommand.cmake.in b/Modules/ExternalProject/customcommand.cmake.in
new file mode 100644
index 0000000..d41f31b
--- /dev/null
+++ b/Modules/ExternalProject/customcommand.cmake.in
@@ -0,0 +1,8 @@
+
+execute_process(
+  COMMAND @this_command@
+  WORKING_DIRECTORY "@work_dir@"
+  RESULT_VARIABLE result
+  ${capture_output}
+)
+_ep_command_check_result(result)
diff --git a/Modules/ExternalProject/customcommand_preamble.cmake.in b/Modules/ExternalProject/customcommand_preamble.cmake.in
new file mode 100644
index 0000000..ae4fec6
--- /dev/null
+++ b/Modules/ExternalProject/customcommand_preamble.cmake.in
@@ -0,0 +1,8 @@
+# Distributed under the OSI-approved BSD 3-Clause License.  See accompanying
+# file Copyright.txt or https://cmake.org/licensing for details.
+
+cmake_minimum_required(VERSION 3.5)
+
+set(quiet "@quiet@")
+set(script_dir "@CMAKE_CURRENT_FUNCTION_LIST_DIR@/ExternalProject")
+include(${script_dir}/captured_process_setup.cmake)
diff --git a/Modules/ExternalProject/download.cmake.in b/Modules/ExternalProject/download.cmake.in
index 6ef4eb1..c8d2f28 100644
--- a/Modules/ExternalProject/download.cmake.in
+++ b/Modules/ExternalProject/download.cmake.in
@@ -3,13 +3,17 @@
 
 cmake_minimum_required(VERSION 3.5)
 
+set(quiet "@quiet@")
+set(script_dir "@CMAKE_CURRENT_FUNCTION_LIST_DIR@/ExternalProject")
+include(${script_dir}/captured_process_setup.cmake)
+
 function(check_file_hash has_hash hash_is_good)
   if("${has_hash}" STREQUAL "")
-    message(FATAL_ERROR "has_hash Can't be empty")
+    _ep_message_quiet_capture(FATAL_ERROR "has_hash Can't be empty")
   endif()
 
   if("${hash_is_good}" STREQUAL "")
-    message(FATAL_ERROR "hash_is_good Can't be empty")
+    _ep_message_quiet_capture(FATAL_ERROR "hash_is_good Can't be empty")
   endif()
 
   if("@ALGO@" STREQUAL "")
@@ -21,18 +25,20 @@ function(check_file_hash has_hash hash_is_good)
 
   set("${has_hash}" TRUE PARENT_SCOPE)
 
-  message(STATUS "verifying file...
+  _ep_message_quiet_capture(STATUS "verifying file...
        file='@LOCAL@'")
+  set(accumulated_output "${accumulated_output}" PARENT_SCOPE)
 
   file("@ALGO@" "@LOCAL@" actual_value)
 
   if(NOT "${actual_value}" STREQUAL "@EXPECT_VALUE@")
     set("${hash_is_good}" FALSE PARENT_SCOPE)
-    message(STATUS "@ALGO@ hash of
+    _ep_message_quiet_capture(STATUS "@ALGO@ hash of
     @LOCAL@
   does not match expected value
     expected: '@EXPECT_VALUE@'
       actual: '${actual_value}'")
+    set(accumulated_output "${accumulated_output}" PARENT_SCOPE)
   else()
     set("${hash_is_good}" TRUE PARENT_SCOPE)
   endif()
@@ -44,7 +50,8 @@ function(sleep_before_download attempt)
   endif()
 
   if(attempt EQUAL 1)
-    message(STATUS "Retrying...")
+    _ep_message_quiet_capture(STATUS "Retrying...")
+    set(accumulated_output "${accumulated_output}" PARENT_SCOPE)
     return()
   endif()
 
@@ -66,7 +73,10 @@ function(sleep_before_download attempt)
     set(sleep_seconds 1200)
   endif()
 
-  message(STATUS "Retry after ${sleep_seconds} seconds (attempt #${attempt}) ...")
+  _ep_message_quiet_capture(STATUS
+    "Retry after ${sleep_seconds} seconds (attempt #${attempt}) ..."
+  )
+  set(accumulated_output "${accumulated_output}" PARENT_SCOPE)
 
   execute_process(COMMAND "${CMAKE_COMMAND}" -E sleep "${sleep_seconds}")
 endfunction()
@@ -84,18 +94,22 @@ function(download_and_verify)
     check_file_hash(has_hash hash_is_good)
     if(has_hash)
       if(hash_is_good)
-        message(STATUS
+        _ep_message_quiet_capture(STATUS
 "File already exists and hash match (skip download):
   file='@LOCAL@'
   @ALGO@='@EXPECT_VALUE@'"
         )
+        set(accumulated_output "${accumulated_output}" PARENT_SCOPE)
         return()
       else()
-        message(STATUS "File already exists but hash mismatch. Removing...")
+        _ep_message_quiet_capture(STATUS
+          "File already exists but hash mismatch. Removing..."
+        )
+        set(accumulated_output "${accumulated_output}" PARENT_SCOPE)
         file(REMOVE "@LOCAL@")
       endif()
     else()
-      message(STATUS
+      _ep_message_quiet_capture(STATUS
 "File already exists but no hash specified (use URL_HASH):
   file='@LOCAL@'
 Old file will be removed and new file downloaded from URL."
@@ -106,11 +120,12 @@ Old file will be removed and new file downloaded from URL."
 
   set(retry_number 5)
 
-  message(STATUS "Downloading...
+  _ep_message_quiet_capture(STATUS "Downloading...
    dst='@LOCAL@'
    timeout='@TIMEOUT_MSG@'
    inactivity timeout='@INACTIVITY_TIMEOUT_MSG@'"
   )
+  set(accumulated_output "${accumulated_output}" PARENT_SCOPE)
   set(download_retry_codes 7 6 8 15)
   set(skip_url_list)
   set(status_code)
@@ -120,7 +135,8 @@ Old file will be removed and new file downloaded from URL."
     endif()
     foreach(url @REMOTE@)
       if(NOT url IN_LIST skip_url_list)
-        message(STATUS "Using src='${url}'")
+        _ep_message_quiet_capture(STATUS "Using src='${url}'")
+        set(accumulated_output "${accumulated_output}" PARENT_SCOPE)
 
         @TLS_VERIFY_CODE@
         @TLS_CAINFO_CODE@
@@ -145,10 +161,12 @@ Old file will be removed and new file downloaded from URL."
         if(status_code EQUAL 0)
           check_file_hash(has_hash hash_is_good)
           if(has_hash AND NOT hash_is_good)
-            message(STATUS "Hash mismatch, removing...")
+            _ep_message_quiet_capture(STATUS "Hash mismatch, removing...")
+            set(accumulated_output "${accumulated_output}" PARENT_SCOPE)
             file(REMOVE "@LOCAL@")
           else()
-            message(STATUS "Downloading... done")
+            _ep_message_quiet_capture(STATUS "Downloading... done")
+            set(accumulated_output "${accumulated_output}" PARENT_SCOPE)
             return()
           endif()
         else()
@@ -171,7 +189,7 @@ Old file will be removed and new file downloaded from URL."
     endforeach()
   endforeach()
 
-  message(FATAL_ERROR
+  _ep_message_quiet_capture(FATAL_ERROR
 "Each download failed!
   ${logFailedURLs}
   "
diff --git a/Modules/ExternalProject/extractfile.cmake.in b/Modules/ExternalProject/extractfile.cmake.in
index d9e07f1..d46de73 100644
--- a/Modules/ExternalProject/extractfile.cmake.in
+++ b/Modules/ExternalProject/extractfile.cmake.in
@@ -3,17 +3,24 @@
 
 cmake_minimum_required(VERSION 3.5)
 
+set(quiet "@quiet@")
+set(script_dir "@CMAKE_CURRENT_FUNCTION_LIST_DIR@/ExternalProject")
+include(${script_dir}/captured_process_setup.cmake)
+
 # Make file names absolute:
 #
 get_filename_component(filename "@filename@" ABSOLUTE)
 get_filename_component(directory "@directory@" ABSOLUTE)
 
-message(STATUS "extracting...
+_ep_message_quiet_capture(STATUS "extracting...
      src='${filename}'
-     dst='${directory}'")
+     dst='${directory}'"
+)
 
 if(NOT EXISTS "${filename}")
-  message(FATAL_ERROR "File to extract does not exist: '${filename}'")
+  _ep_message_quiet_capture(FATAL_ERROR
+    "File to extract does not exist: '${filename}'"
+  )
 endif()
 
 # Prepare a space for extracting:
@@ -27,20 +34,23 @@ file(MAKE_DIRECTORY "${ut_dir}")
 
 # Extract it:
 #
-message(STATUS "extracting... [tar @args@]")
+_ep_message_quiet_capture(STATUS "extracting... [tar @args@]")
 execute_process(COMMAND ${CMAKE_COMMAND} -E tar @args@ ${filename}
   WORKING_DIRECTORY ${ut_dir}
-  RESULT_VARIABLE rv)
+  RESULT_VARIABLE rv
+  ${capture_output}
+)
+_ep_accumulate_captured_output()
 
 if(NOT rv EQUAL 0)
-  message(STATUS "extracting... [error clean up]")
+  _ep_message_quiet_capture(STATUS "extracting... [error clean up]")
   file(REMOVE_RECURSE "${ut_dir}")
-  message(FATAL_ERROR "Extract of '${filename}' failed")
+  _ep_message_quiet_capture(FATAL_ERROR "Extract of '${filename}' failed")
 endif()
 
 # Analyze what came out of the tar file:
 #
-message(STATUS "extracting... [analysis]")
+_ep_message_quiet_capture(STATUS "extracting... [analysis]")
 file(GLOB contents "${ut_dir}/*")
 list(REMOVE_ITEM contents "${ut_dir}/.DS_Store")
 list(LENGTH contents n)
@@ -50,14 +60,14 @@ endif()
 
 # Move "the one" directory to the final directory:
 #
-message(STATUS "extracting... [rename]")
+_ep_message_quiet_capture(STATUS "extracting... [rename]")
 file(REMOVE_RECURSE ${directory})
 get_filename_component(contents ${contents} ABSOLUTE)
 file(RENAME ${contents} ${directory})
 
 # Clean up:
 #
-message(STATUS "extracting... [clean up]")
+_ep_message_quiet_capture(STATUS "extracting... [clean up]")
 file(REMOVE_RECURSE "${ut_dir}")
 
-message(STATUS "extracting... done")
+_ep_message_quiet_capture(STATUS "extracting... done")
diff --git a/Modules/ExternalProject/gitclone.cmake.in b/Modules/ExternalProject/gitclone.cmake.in
index 5e5c415..a2e900c 100644
--- a/Modules/ExternalProject/gitclone.cmake.in
+++ b/Modules/ExternalProject/gitclone.cmake.in
@@ -3,57 +3,81 @@
 
 cmake_minimum_required(VERSION 3.5)
 
+set(quiet "@quiet@")
+set(script_dir "@CMAKE_CURRENT_FUNCTION_LIST_DIR@/ExternalProject")
+include(${script_dir}/captured_process_setup.cmake)
+
 if(NOT "@gitclone_infofile@" IS_NEWER_THAN "@gitclone_stampfile@")
-  message(STATUS "Avoiding repeated git clone, stamp file is up to date: '@gitclone_stampfile@'")
+  if(NOT quiet)
+    message(STATUS
+      "Avoiding repeated git clone, stamp file is up to date: "
+      "'@gitclone_stampfile@'"
+    )
+  endif()
   return()
 endif()
 
 execute_process(
   COMMAND ${CMAKE_COMMAND} -E rm -rf "@source_dir@"
   RESULT_VARIABLE error_code
-  )
-if(error_code)
-  message(FATAL_ERROR "Failed to remove directory: '@source_dir@'")
-endif()
+  ${capture_output}
+)
+_ep_command_check_result(
+  error_code "Failed to remove directory: '@source_dir@'"
+)
 
 # try the clone 3 times in case there is an odd git clone issue
 set(error_code 1)
 set(number_of_tries 0)
 while(error_code AND number_of_tries LESS 3)
+  # If you are seeing the following call hang and you have QUIET enabled, try
+  # turning QUIET off to show any output immediately. The command may be
+  # blocking while waiting for user input (e.g. a password to a SSH key).
   execute_process(
-    COMMAND "@git_EXECUTABLE@" @git_options@ clone @git_clone_options@ "@git_repository@" "@src_name@"
+    COMMAND "@git_EXECUTABLE@" @git_options@
+            clone @git_clone_options@ "@git_repository@" "@src_name@"
     WORKING_DIRECTORY "@work_dir@"
     RESULT_VARIABLE error_code
-    )
+    ${capture_output}
+  )
+  if(NOT "${out_var}" STREQUAL "")
+    string(APPEND accumulated_output "${out_var}\n")
+  endif()
   math(EXPR number_of_tries "${number_of_tries} + 1")
 endwhile()
 if(number_of_tries GREATER 1)
-  message(STATUS "Had to git clone more than once:
-          ${number_of_tries} times.")
-endif()
-if(error_code)
-  message(FATAL_ERROR "Failed to clone repository: '@git_repository@'")
+  set(msg "Had to git clone more than once: ${number_of_tries} times.")
+  if(quiet)
+    string(APPEND accumulated_output "${msg}\n")
+  else()
+    message(STATUS "${msg}")
+  endif()
 endif()
+_ep_command_check_result(
+  error_code "Failed to clone repository: '@git_repository@'"
+)
 
 execute_process(
-  COMMAND "@git_EXECUTABLE@" @git_options@ checkout "@git_tag@" @git_checkout_explicit--@
+  COMMAND "@git_EXECUTABLE@" @git_options@
+          checkout "@git_tag@" @git_checkout_explicit--@
   WORKING_DIRECTORY "@work_dir@/@src_name@"
   RESULT_VARIABLE error_code
-  )
-if(error_code)
-  message(FATAL_ERROR "Failed to checkout tag: '@git_tag@'")
-endif()
+  ${capture_output}
+)
+_ep_command_check_result(error_code "Failed to checkout tag: '@git_tag@'")
 
 set(init_submodules @init_submodules@)
 if(init_submodules)
   execute_process(
-    COMMAND "@git_EXECUTABLE@" @git_options@ submodule update @git_submodules_recurse@ --init @git_submodules@
+    COMMAND "@git_EXECUTABLE@" @git_options@
+            submodule update @git_submodules_recurse@ --init @git_submodules@
     WORKING_DIRECTORY "@work_dir@/@src_name@"
     RESULT_VARIABLE error_code
-    )
-endif()
-if(error_code)
-  message(FATAL_ERROR "Failed to update submodules in: '@work_dir@/@src_name@'")
+    ${capture_output}
+  )
+  _ep_command_check_result(
+    error_code "Failed to update submodules in: '@work_dir@/@src_name@'"
+  )
 endif()
 
 # Complete success, update the script-last-run stamp file:
@@ -61,7 +85,8 @@ endif()
 execute_process(
   COMMAND ${CMAKE_COMMAND} -E copy "@gitclone_infofile@" "@gitclone_stampfile@"
   RESULT_VARIABLE error_code
-  )
-if(error_code)
-  message(FATAL_ERROR "Failed to copy script-last-run stamp file: '@gitclone_stampfile@'")
-endif()
+  ${capture_output}
+)
+_ep_command_check_result(
+  error_code "Failed to copy script-last-run stamp file: '@gitclone_stampfile@'"
+)
diff --git a/Modules/ExternalProject/gitupdate.cmake.in b/Modules/ExternalProject/gitupdate.cmake.in
index 7033918..fc2a6ab 100644
--- a/Modules/ExternalProject/gitupdate.cmake.in
+++ b/Modules/ExternalProject/gitupdate.cmake.in
@@ -3,6 +3,10 @@
 
 cmake_minimum_required(VERSION 3.5)
 
+set(quiet "@quiet@")
+set(script_dir "@CMAKE_CURRENT_FUNCTION_LIST_DIR@/ExternalProject")
+include(${script_dir}/captured_process_setup.cmake)
+
 function(get_hash_for_ref ref out_var err_var)
   execute_process(
     COMMAND "@git_EXECUTABLE@" rev-parse "${ref}"
@@ -49,7 +53,7 @@ elseif(show_ref_output MATCHES "^[a-z0-9]+[ \\t]+refs/tags/")
   # FIXME: We should provide an option to always fetch for this case
   get_hash_for_ref("@git_tag@" tag_sha error_msg)
   if(tag_sha STREQUAL head_sha)
-    message(VERBOSE "Already at requested tag: ${tag_sha}")
+    _ep_message_quiet_capture(VERBOSE "Already at requested tag: ${tag_sha}")
     return()
   endif()
 
@@ -65,7 +69,7 @@ else()
   get_hash_for_ref("@git_tag@" tag_sha error_msg)
   if(tag_sha STREQUAL head_sha)
     # Have the right commit checked out already
-    message(VERBOSE "Already at requested ref: ${tag_sha}")
+    _ep_message_quiet_capture(VERBOSE "Already at requested ref: ${tag_sha}")
     return()
 
   elseif(tag_sha STREQUAL "")
@@ -76,7 +80,7 @@ else()
     set(fetch_required YES)
     set(checkout_name "@git_tag@")
     if(NOT error_msg STREQUAL "")
-      message(VERBOSE "${error_msg}")
+      _ep_message_quiet_capture(VERBOSE "${error_msg}")
     endif()
 
   else()
@@ -86,18 +90,22 @@ else()
     set(fetch_required NO)
     set(checkout_name "@git_tag@")
     if(NOT error_msg STREQUAL "")
-      message(WARNING "${error_msg}")
+      _ep_message_quiet_capture(WARNING "${error_msg}")
     endif()
 
   endif()
 endif()
 
 if(fetch_required)
-  message(VERBOSE "Fetching latest from the remote @git_remote_name@")
+  _ep_message_quiet_capture(VERBOSE "Fetching latest from the remote @git_remote_name@")
   execute_process(
     COMMAND "@git_EXECUTABLE@" fetch --tags --force "@git_remote_name@"
     WORKING_DIRECTORY "@work_dir@"
-    COMMAND_ERROR_IS_FATAL ANY
+    RESULT_VARIABLE error_code
+    ${capture_output}
+  )
+  _ep_command_check_result(
+    error_code "Failed to fetch from the remote @git_remote_name@'"
   )
 endif()
 
@@ -128,12 +136,15 @@ if(git_update_strategy MATCHES "^REBASE(_CHECKOUT)?$")
 
   else()
     execute_process(
-      COMMAND "@git_EXECUTABLE@" for-each-ref "--format='%(upstream:short)'" "${current_branch}"
+      COMMAND "@git_EXECUTABLE@" for-each-ref
+              "--format='%(upstream:short)'" "${current_branch}"
       WORKING_DIRECTORY "@work_dir@"
+      RESULT_VARIABLE error_code  # There is no error if no upstream is set
       OUTPUT_VARIABLE upstream_branch
       OUTPUT_STRIP_TRAILING_WHITESPACE
-      COMMAND_ERROR_IS_FATAL ANY  # There is no error if no upstream is set
+      ${capture_error_only}
     )
+    _ep_command_check_result(error_code)
     if(NOT upstream_branch STREQUAL checkout_name)
       # Not safe to rebase when asked to checkout a different branch to the one
       # we are tracking. If we did rebase, we could end up with arbitrary
@@ -145,7 +156,9 @@ if(git_update_strategy MATCHES "^REBASE(_CHECKOUT)?$")
 
   endif()
 elseif(NOT git_update_strategy STREQUAL "CHECKOUT")
-  message(FATAL_ERROR "Unsupported git update strategy: ${git_update_strategy}")
+  _ep_message_quiet_capture(FATAL_ERROR
+    "Unsupported git update strategy: ${git_update_strategy}"
+  )
 endif()
 
 
@@ -155,10 +168,9 @@ execute_process(
   WORKING_DIRECTORY "@work_dir@"
   RESULT_VARIABLE error_code
   OUTPUT_VARIABLE repo_status
+  ${capture_error_only}
 )
-if(error_code)
-  message(FATAL_ERROR "Failed to get the status")
-endif()
+_ep_command_check_result(error_code "Failed to get the status")
 string(LENGTH "${repo_status}" need_stash)
 
 # If not in clean state, stash changes in order to be able to perform a
@@ -167,16 +179,20 @@ if(need_stash)
   execute_process(
     COMMAND "@git_EXECUTABLE@" stash save @git_stash_save_options@
     WORKING_DIRECTORY "@work_dir@"
-    COMMAND_ERROR_IS_FATAL ANY
+    RESULT_VARIABLE error_code
+    ${capture_output}
   )
+  _ep_command_check_result(error_code)
 endif()
 
 if(git_update_strategy STREQUAL "CHECKOUT")
   execute_process(
     COMMAND "@git_EXECUTABLE@" checkout "${checkout_name}"
     WORKING_DIRECTORY "@work_dir@"
-    COMMAND_ERROR_IS_FATAL ANY
+    RESULT_VARIABLE error_code
+    ${capture_output}
   )
+  _ep_command_check_result(error_code)
 else()
   execute_process(
     COMMAND "@git_EXECUTABLE@" rebase "${checkout_name}"
@@ -198,12 +214,14 @@ else()
         execute_process(
           COMMAND "@git_EXECUTABLE@" stash pop --index --quiet
           WORKING_DIRECTORY "@work_dir@"
-          )
+        )
       endif()
-      message(FATAL_ERROR "\nFailed to rebase in: '@work_dir@'."
-                          "\nOutput from the attempted rebase follows:"
-                          "\n${rebase_output}"
-                          "\n\nYou will have to resolve the conflicts manually")
+      _ep_message_quiet_capture(FATAL_ERROR
+        "\nFailed to rebase in: '@work_dir@'."
+        "\nOutput from the attempted rebase follows:"
+        "\n${rebase_output}"
+        "\n\nYou will have to resolve the conflicts manually"
+      )
     endif()
 
     # Fall back to checkout. We create an annotated tag so that the user
@@ -215,21 +233,27 @@ else()
     set(tag_name _cmake_ExternalProject_moved_from_here_${tag_timestamp}Z)
     set(error_log_file ${CMAKE_CURRENT_LIST_DIR}/rebase_error_${tag_timestamp}Z.log)
     file(WRITE ${error_log_file} "${rebase_output}")
-    message(WARNING "Rebase failed, output has been saved to ${error_log_file}"
-                    "\nFalling back to checkout, previous commit tagged as ${tag_name}")
+    _ep_message_quiet_capture(WARNING
+      "Rebase failed, output has been saved to ${error_log_file}"
+      "\nFalling back to checkout, previous commit tagged as ${tag_name}"
+    )
     execute_process(
       COMMAND "@git_EXECUTABLE@" tag -a
               -m "ExternalProject attempting to move from here to ${checkout_name}"
               ${tag_name}
       WORKING_DIRECTORY "@work_dir@"
-      COMMAND_ERROR_IS_FATAL ANY
+      RESULT_VARIABLE error_code
+      ${capture_output}
     )
+    _ep_command_check_result(error_code)
 
     execute_process(
       COMMAND "@git_EXECUTABLE@" checkout "${checkout_name}"
       WORKING_DIRECTORY "@work_dir@"
-      COMMAND_ERROR_IS_FATAL ANY
+      RESULT_VARIABLE error_code
+      ${capture_output}
     )
+    _ep_command_check_result(error_code)
   endif()
 endif()
 
@@ -239,30 +263,42 @@ if(need_stash)
     COMMAND "@git_EXECUTABLE@" stash pop --index --quiet
     WORKING_DIRECTORY "@work_dir@"
     RESULT_VARIABLE error_code
-    )
+    ${capture_output}
+  )
+  _ep_accumulate_captured_output()
   if(error_code)
     # Stash pop --index failed: Try again dropping the index
     execute_process(
       COMMAND "@git_EXECUTABLE@" reset --hard --quiet
       WORKING_DIRECTORY "@work_dir@"
+      ${capture_output}
     )
+    _ep_accumulate_captured_output()
     execute_process(
       COMMAND "@git_EXECUTABLE@" stash pop --quiet
       WORKING_DIRECTORY "@work_dir@"
       RESULT_VARIABLE error_code
+      ${capture_output}
     )
+    _ep_accumulate_captured_output()
     if(error_code)
       # Stash pop failed: Restore previous state.
       execute_process(
         COMMAND "@git_EXECUTABLE@" reset --hard --quiet ${head_sha}
         WORKING_DIRECTORY "@work_dir@"
+        ${capture_output}
       )
+      _ep_accumulate_captured_output()
       execute_process(
         COMMAND "@git_EXECUTABLE@" stash pop --index --quiet
         WORKING_DIRECTORY "@work_dir@"
+        ${capture_output}
+      )
+      _ep_accumulate_captured_output()
+      _ep_message_quiet_capture(FATAL_ERROR
+        "Failed to unstash changes in: '@work_dir@'.\n"
+        "You will have to resolve the conflicts manually"
       )
-      message(FATAL_ERROR "\nFailed to unstash changes in: '@work_dir@'."
-                          "\nYou will have to resolve the conflicts manually")
     endif()
   endif()
 endif()
@@ -272,6 +308,8 @@ if(init_submodules)
   execute_process(
     COMMAND "@git_EXECUTABLE@" submodule update @git_submodules_recurse@ --init @git_submodules@
     WORKING_DIRECTORY "@work_dir@"
-    COMMAND_ERROR_IS_FATAL ANY
+    RESULT_VARIABLE error_code
+    ${capture_output}
   )
+  _ep_command_check_result(error_code)
 endif()
diff --git a/Modules/ExternalProject/hgclone.cmake.in b/Modules/ExternalProject/hgclone.cmake.in
index 09395cc..5561955 100644
--- a/Modules/ExternalProject/hgclone.cmake.in
+++ b/Modules/ExternalProject/hgclone.cmake.in
@@ -3,43 +3,56 @@
 
 cmake_minimum_required(VERSION 3.5)
 
+set(quiet "@quiet@")
+set(script_dir "@CMAKE_CURRENT_FUNCTION_LIST_DIR@/ExternalProject")
+include(${script_dir}/captured_process_setup.cmake)
+
 if(NOT "@hgclone_infofile@" IS_NEWER_THAN "@hgclone_stampfile@")
-  message(STATUS "Avoiding repeated hg clone, stamp file is up to date: '@hgclone_stampfile@'")
+  if(NOT quiet)
+    message(STATUS
+      "Avoiding repeated hg clone, stamp file is up to date: "
+      "'@hgclone_stampfile@'"
+    )
+  endif()
   return()
 endif()
 
 execute_process(
   COMMAND ${CMAKE_COMMAND} -E rm -rf "@source_dir@"
   RESULT_VARIABLE error_code
-  )
-if(error_code)
-  message(FATAL_ERROR "Failed to remove directory: '@source_dir@'")
-endif()
+  ${capture_output}
+)
+_ep_command_check_result(
+  error_code "Failed to remove directory: '@source_dir@'"
+)
 
 execute_process(
   COMMAND "@hg_EXECUTABLE@" clone -U "@hg_repository@" "@src_name@"
   WORKING_DIRECTORY "@work_dir@"
   RESULT_VARIABLE error_code
-  )
-if(error_code)
-  message(FATAL_ERROR "Failed to clone repository: '@hg_repository@'")
-endif()
+  ${capture_output}
+)
+_ep_command_check_result(
+  error_code "Failed to clone repository: '@hg_repository@'"
+)
 
 execute_process(
   COMMAND "@hg_EXECUTABLE@" update @hg_tag@
   WORKING_DIRECTORY "@work_dir@/@src_name@"
   RESULT_VARIABLE error_code
-  )
-if(error_code)
-  message(FATAL_ERROR "Failed to checkout tag: '@hg_tag@'")
-endif()
+  ${capture_output}
+)
+_ep_command_check_result(
+  error_code "Failed to checkout tag: '@hg_tag@'"
+)
 
 # Complete success, update the script-last-run stamp file:
 #
 execute_process(
   COMMAND ${CMAKE_COMMAND} -E copy "@hgclone_infofile@" "@hgclone_stampfile@"
   RESULT_VARIABLE error_code
-  )
-if(error_code)
-  message(FATAL_ERROR "Failed to copy script-last-run stamp file: '@hgclone_stampfile@'")
-endif()
+  ${capture_output}
+)
+_ep_command_check_result(
+  error_code "Failed to copy script-last-run stamp file: '@hgclone_stampfile@'"
+)
diff --git a/Modules/ExternalProject/hgupdate.cmake.in b/Modules/ExternalProject/hgupdate.cmake.in
index f88e1ee..a82a819 100644
--- a/Modules/ExternalProject/hgupdate.cmake.in
+++ b/Modules/ExternalProject/hgupdate.cmake.in
@@ -3,14 +3,22 @@
 
 cmake_minimum_required(VERSION 3.19)
 
+set(quiet "@quiet@")
+set(script_dir "@CMAKE_CURRENT_FUNCTION_LIST_DIR@/ExternalProject")
+include(${script_dir}/captured_process_setup.cmake)
+
 execute_process(
   COMMAND "@hg_EXECUTABLE@" pull
-  COMMAND_ERROR_IS_FATAL ANY
   WORKING_DIRECTORY "@work_dir@"
+  RESULT_VARIABLE error_code
+  ${capture_output}
 )
+_ep_command_check_result(error_code)
 
 execute_process(
   COMMAND "@hg_EXECUTABLE@" update @hg_tag@
-  COMMAND_ERROR_IS_FATAL ANY
   WORKING_DIRECTORY "@work_dir@"
+  RESULT_VARIABLE error_code
+  ${capture_output}
 )
+_ep_command_check_result(error_code)
diff --git a/Modules/ExternalProject/verify.cmake.in b/Modules/ExternalProject/verify.cmake.in
index f37059b..cd34ba9 100644
--- a/Modules/ExternalProject/verify.cmake.in
+++ b/Modules/ExternalProject/verify.cmake.in
@@ -3,6 +3,10 @@
 
 cmake_minimum_required(VERSION 3.5)
 
+set(quiet "@quiet@")
+set(script_dir "@CMAKE_CURRENT_FUNCTION_LIST_DIR@/ExternalProject")
+include(${script_dir}/captured_process_setup.cmake)
+
 if("@LOCAL@" STREQUAL "")
   message(FATAL_ERROR "LOCAL can't be empty")
 endif()
@@ -13,22 +17,27 @@ endif()
 
 function(do_verify)
   if("@ALGO@" STREQUAL "")
-    message(WARNING "File will not be verified since no URL_HASH specified")
+    _ep_message_quiet_capture(WARNING
+      "File will not be verified since no URL_HASH specified"
+    )
+    set(accumulated_output "${accumulated_output}" PARENT_SCOPE)
     return()
   endif()
 
   if("@EXPECT_VALUE@" STREQUAL "")
-    message(FATAL_ERROR "EXPECT_VALUE can't be empty")
+    _ep_message_quiet_capture(FATAL_ERROR "EXPECT_VALUE can't be empty")
   endif()
 
-  message(STATUS
+  _ep_message_quiet_capture(STATUS
 "verifying file...
-     file='@LOCAL@'")
+     file='@LOCAL@'"
+  )
+  set(accumulated_output "${accumulated_output}" PARENT_SCOPE)
 
   file("@ALGO@" "@LOCAL@" actual_value)
 
   if(NOT "${actual_value}" STREQUAL "@EXPECT_VALUE@")
-    message(FATAL_ERROR
+    _ep_message_quiet_capture(FATAL_ERROR
 "error: @ALGO@ hash of
   @LOCAL@
 does not match expected value
@@ -37,7 +46,8 @@ does not match expected value
 ")
   endif()
 
-  message(STATUS "verifying file... done")
+  _ep_message_quiet_capture(STATUS "verifying file... done")
+  set(accumulated_output "${accumulated_output}" PARENT_SCOPE)
 endfunction()
 
 do_verify()