From 4e7e6928cb9256ceb7529c1e2f42d7ae8d65329f Mon Sep 17 00:00:00 2001
From: Tyler Yankee <tyler.yankee@kitware.com>
Date: Thu, 5 Feb 2026 15:26:54 -0500
Subject: [PATCH] install: Fix bugs around empty directories

The conversion between internal list representations led to empty
directories being left behind when processing generator expressions in
the destination. This ensures the intended behavior of creating a
destination directory but installing nothing into it when no <dirs> are
specified, regardless of the presence or absence of genex.

Further, an explicit empty string <dir> (via `""`, or an undefined
variable) led to the current source directory being expanded as a file
to install, leading to the generation of an infinitely recursive
install command. Disallow this behavior, forcing projects to explicitly
specify the current source directory if that is their intention.

Fixes: #27568
---
 Source/cmInstallCommand.cxx                   |  5 +++--
 Source/cmInstallDirectoryGenerator.cxx        | 20 ++++++++++++++-----
 .../install/DIRECTORY-PATTERN-all-check.cmake |  2 +-
 .../RunCMake/install/DIRECTORY-PATTERN.cmake  | 10 ++++++++++
 4 files changed, 29 insertions(+), 8 deletions(-)

diff --git a/Source/cmInstallCommand.cxx b/Source/cmInstallCommand.cxx
index 2d632e7dbe..1449f3483c 100644
--- a/Source/cmInstallCommand.cxx
+++ b/Source/cmInstallCommand.cxx
@@ -1791,10 +1791,11 @@ bool HandleDirectoryMode(std::vector<std::string> const& args,
       exclude_from_all = true;
       doing = DoingNone;
     } else if (doing == DoingDirs) {
-      // Convert this directory to a full path.
+      // If the given directory is not a full path, convert it to one by
+      // assuming it's relative to the current source directory.
       std::string dir = args[i];
       std::string::size_type gpos = cmGeneratorExpression::Find(dir);
-      if (gpos != 0 && !cmSystemTools::FileIsFullPath(dir)) {
+      if (!dir.empty() && gpos != 0 && !cmSystemTools::FileIsFullPath(dir)) {
         dir =
           cmStrCat(helper.Makefile->GetCurrentSourceDirectory(), '/', args[i]);
       }
diff --git a/Source/cmInstallDirectoryGenerator.cxx b/Source/cmInstallDirectoryGenerator.cxx
index a388d917d8..cf1fac91fc 100644
--- a/Source/cmInstallDirectoryGenerator.cxx
+++ b/Source/cmInstallDirectoryGenerator.cxx
@@ -2,6 +2,7 @@
    file LICENSE.rst or https://cmake.org/licensing for details.  */
 #include "cmInstallDirectoryGenerator.h"
 
+#include <algorithm>
 #include <utility>
 
 #include "cmGeneratorExpression.h"
@@ -54,6 +55,13 @@ bool cmInstallDirectoryGenerator::Compute(cmLocalGenerator* lg)
 std::vector<std::string> cmInstallDirectoryGenerator::GetDirectories(
   std::string const& config) const
 {
+  // If given only empty directories, collapse into a single specification to
+  // avoid redundant calls. This supports the use case of installing an empty
+  // directory into a destination when a directory is not specified.
+  if (std::all_of(this->Directories.begin(), this->Directories.end(),
+                  [](std::string const& d) { return d.empty(); })) {
+    return std::vector<std::string>{ "" };
+  }
   cmList directories;
   if (this->ActionsPerConfig) {
     for (std::string const& f : this->Directories) {
@@ -81,11 +89,13 @@ void cmInstallDirectoryGenerator::GenerateScriptForConfig(
 {
   std::vector<std::string> dirs = this->GetDirectories(config);
 
-  // Make sure all dirs have absolute paths.
-  cmMakefile const& mf = *this->LocalGenerator->GetMakefile();
-  for (std::string& d : dirs) {
-    if (!cmSystemTools::FileIsFullPath(d)) {
-      d = cmStrCat(mf.GetCurrentSourceDirectory(), '/', d);
+  if (!(dirs.size() == 1 && dirs[0].empty())) {
+    // Make sure all dirs have absolute paths.
+    cmMakefile const& mf = *this->LocalGenerator->GetMakefile();
+    for (std::string& d : dirs) {
+      if (!cmSystemTools::FileIsFullPath(d)) {
+        d = cmStrCat(mf.GetCurrentSourceDirectory(), '/', d);
+      }
     }
   }
 
diff --git a/Tests/RunCMake/install/DIRECTORY-PATTERN-all-check.cmake b/Tests/RunCMake/install/DIRECTORY-PATTERN-all-check.cmake
index 7a20edc364..504d1fde35 100644
--- a/Tests/RunCMake/install/DIRECTORY-PATTERN-all-check.cmake
+++ b/Tests/RunCMake/install/DIRECTORY-PATTERN-all-check.cmake
@@ -1 +1 @@
-check_installed([[^dir1;dir1/empty\.c;dir1/empty\.h;dir2;dir2/pattern;dir2/pattern/empty\.txt;dir3;dir3/empty\.c;dir3/empty\.h;dir3/empty\.txt;dir4;dir4/empty\.c;dir4/empty\.h;dir4/empty\.txt;empty$]])
+check_installed([[^dir1;dir1/empty\.c;dir1/empty\.h;dir2;dir2/pattern;dir2/pattern/empty\.txt;dir3;dir3/empty\.c;dir3/empty\.h;dir3/empty\.txt;dir4;dir4/empty\.c;dir4/empty\.h;dir4/empty\.txt;empty;empty_str;empty_with_genex$]])
diff --git a/Tests/RunCMake/install/DIRECTORY-PATTERN.cmake b/Tests/RunCMake/install/DIRECTORY-PATTERN.cmake
index 74d804351e..61007ac44f 100644
--- a/Tests/RunCMake/install/DIRECTORY-PATTERN.cmake
+++ b/Tests/RunCMake/install/DIRECTORY-PATTERN.cmake
@@ -34,3 +34,13 @@ install(
   DIRECTORY
   DESTINATION empty
   )
+
+install(
+  DIRECTORY "" ""
+  DESTINATION empty_str
+  )
+
+install(
+  DIRECTORY
+  DESTINATION $<LOWER_CASE:EMPTY_WITH_GENEX>
+  )