fix: use tmp dir if $HOME isn't writable

src/alire/alire-platforms-common.adb

@@ -1,4 +1,5 @@
 with AAA.Enum_Tools;
+with GNAT.OS_Lib;
 
 with Alire.OS_Lib.Subprocess;
 
@@ -71,4 +72,27 @@ package body Alire.Platforms.Common is
       end Detect;
    end Machine_Hardware_Name;
 
+   ----------------------
+   -- Unix_Home_Folder --
+   ----------------------
+
+   function Unix_Home_Folder return String
+   is
+      Home_Var      : constant String  := OS_Lib.Getenv ("HOME", "unset");
+      Maybe_Windows : constant Boolean := Home_Var = "unset"
+         and then GNAT.OS_Lib.Directory_Separator = '\';
+   begin
+      if Maybe_Windows then
+         raise Checked_Error with
+            "$HOME is not set, you might be running an"
+            & " `alr` built for a non-Windows OS";
+      elsif Home_Var = "unset" or else
+       not GNAT.OS_Lib.Is_Write_Accessible_File (Home_Var)
+      then
+         return "/tmp";
+      else
+         return Home_Var;
+      end if;
+   end Unix_Home_Folder;
+
 end Alire.Platforms.Common;

src/alire/alire-platforms-common.ads

@@ -19,15 +19,7 @@ private package Alire.Platforms.Common is
    -- Unix_Home_Folder --
    ---------------------
 
-   function Unix_Home_Folder return String
-   is (if OS_Lib.Getenv ("HOME", "unset") = "unset" and then
-       GNAT.OS_Lib.Directory_Separator = '\'
-       then
-          raise Checked_Error with
-            "$HOME is not set, you might be running an"
-       & " `alr` built for a non-Windows OS"
-       else
-          OS_Lib.Getenv ("HOME", Default => "/tmp"));
+   function Unix_Home_Folder return String;
 
    ----------------------
    -- Unix_Temp_Folder --

src/alr/alr-commands-version.adb

@@ -98,6 +98,7 @@ package body Alr.Commands.Version is
 
       Add ("");
       Add ("CONFIGURATION");
+      Add ("home folder:",     Alire.Platforms.Folders.Home);
       Add ("settings folder:", Alire.Settings.Edit.Path);
       Add ("cache folder:",    Alire.Cache.Path);
       Add ("vault folder:",    Paths.Vault.Path);

testsuite/tests/dockerized/misc/no-home/test.py

@@ -0,0 +1,40 @@
+"""
+Verify that alr can run with no HOME set, or with it set to an
+unwritable/nonexistent directory.
+NOTE: This same test is duplicated to be run under docker as a regular user.
+"""
+
+import os
+from drivers.alr import run_alr
+
+
+def check_no_home():
+    # Verify that alr is using the fallback home directory.
+    p = run_alr("--format", "version")
+    assert """{
+    "key": "home folder",
+    "value": "/tmp"
+  }""" in p.out, "Unexpected output: " + p.out
+
+
+# Remove HOME
+os.environ.pop("HOME", None)
+check_no_home()
+
+# Set HOME to a nonexistent directory
+fake_home = "/tmp/fake_home_for_alr"
+assert not os.path.exists(fake_home)
+os.environ["HOME"] = fake_home
+check_no_home()
+
+# Set HOME to an unwritable directory
+# Under our docker, we should be a regular user and this should succeed
+os.makedirs(fake_home)
+os.chmod(fake_home, 0o444)
+check_no_home()
+# Cleanup
+os.chmod(fake_home, 0o777)
+os.rmdir(fake_home)
+
+
+print("SUCCESS")

testsuite/tests/dockerized/misc/no-home/test.yaml

@@ -0,0 +1,7 @@
+driver: docker-wrapper
+build_mode: both
+control:
+    - [SKIP, "skip_docker",  "Test is Docker-only"]
+    - [SKIP, "skip_unix",    "Test is Unix-only"]
+indexes:
+    compiler_only_index: {}

testsuite/tests/misc/no-home/test.py

@@ -0,0 +1,42 @@
+"""
+Verify that alr can run with no HOME set, or with it set to an
+unwritable/nonexistent directory.
+NOTE: This same test is duplicated to be run under docker as a regular user.
+"""
+
+import os
+from drivers.alr import run_alr
+
+
+def check_no_home():
+    # Verify that alr is using the fallback home directory.
+    p = run_alr("--format", "version")
+    assert """{
+    "key": "home folder",
+    "value": "/tmp"
+  }""" in p.out, "Unexpected output: " + p.out
+
+
+# Remove HOME
+os.environ.pop("HOME", None)
+check_no_home()
+
+# Set HOME to a nonexistent directory
+fake_home = "/tmp/fake_home_for_alr"
+assert not os.path.exists(fake_home)
+os.environ["HOME"] = fake_home
+check_no_home()
+
+# Set HOME to an unwritable directory
+# This will fail if running under root, in which case we skip this part.
+# This is tested properly under docker
+if os.getuid() != 0:
+  os.makedirs(fake_home)
+  os.chmod(fake_home, 0o444)
+  check_no_home()
+  # Cleanup
+  os.chmod(fake_home, 0o777)
+  os.rmdir(fake_home)
+
+
+print("SUCCESS")

testsuite/tests/misc/no-home/test.yaml

@@ -0,0 +1,6 @@
+driver: python-script
+build_mode: both
+control:
+    - [SKIP, "skip_unix",    "Test is Unix-only"]
+indexes:
+    compiler_only_index: {}