Gracefully error out upon PATH lookup failure

We perform a PATH lookup of gnatcov to determine the installation
prefix of gnatcov, and in select situations this can fail.

This used to raise an exception during elaboration, it is now changed
to emit a better error message.

Author: leocreuse

testsuite/tests/393-casing_crash/test.opt

@@ -0,0 +1 @@
+!windows-host DEAD Test requires conditions only available on Windows, see #393

testsuite/tests/393-casing_crash/test.py

@@ -0,0 +1,73 @@
+"""
+Check that gnatcov does not crash during elaboration when it fails to find
+itself in PATH.
+
+This requires a fairly specific setup for both path lookup to work in the
+terminal, but not from gnatcov. So far we have managed to do it by
+- Being on a Windows system
+- Having the gnatcov executable being in a case-sensitive directory
+- Modifying the gnatcov.exe filename to change the case of the executable
+  suffix
+- Invoking gnatcov without the executable suffix.
+
+This test aims to re-create these conditions, and thus removes some stuff from
+the environment to make sure we have a the right conditions.
+"""
+
+import os
+
+from e3.os.fs import which
+from e3.fs import mkdir, cp
+
+from SUITE.context import thistest
+from SUITE.cutils import Wdir, contents_of
+from SUITE.tutils import run_and_log
+
+
+tmp = Wdir("tmp_")
+
+# Create an empty directory in which we'll change the case sensitivity and
+# butcher the gnatcov exec.
+gnatcov_install = os.path.abspath("fake_gnatcov_bin")
+mkdir(gnatcov_install)
+
+# Enable file sensitivity for the install dir. This needs to be done while the
+# directory is empty.
+run_and_log(
+    ["fsutil.exe", "file", "setCaseSensitiveInfo", gnatcov_install, "enable"]
+)
+
+# Copy the gnatcov.exe program and change its executable suffix casing
+cp(which("gnatcov.exe"), os.path.join(gnatcov_install, "gnatcov.EXE"))
+
+# Run gnatcov without anything on the environment to make sure we don't pick up
+# the correctly installed gnatcov, not the GNATCOV_PREFIX env var which would
+# bypass the path lookup check. We also go through a cmd indirection to avoid
+# the path lookup that takes place in e3.os.process.Run.
+#
+# We also need gnatcov to be invoked without the exe prefix here as the os
+# search should work, but the command name for gnatcov will still not have
+# the exe suffix.
+crash_log = "xcov.log"
+p = run_and_log(
+    ["cmd", "/c", "gnatcov"],
+    env={"PATH": gnatcov_install},
+    output=crash_log,
+    ignore_environ=True,
+)
+
+thistest.fail_if(
+    p.status == 0,
+    comment="gnatcov did not exit with a failure status",
+)
+
+thistest.fail_if_not_equal(
+    what="Unexpected error message from gnatcov",
+    expected="Could not locate the invoked gnatcov command: <empty string>."
+    " If gnatcov is installed on a case sensitive filesystem or directory,"
+    " ensure the casing as the executable filename is used when invoking the"
+    " program.",
+    actual=contents_of(crash_log).strip(),
+)
+
+thistest.result()

tools/gnatcov/gnatcov.adb

@@ -31,6 +31,19 @@ with Switches;     use Switches;
 procedure GNATcov is
    Use_Gnatcov64 : Boolean := False;
 begin
+   --  Check that gnatcov detected its own installation directory correctly
+
+   if Support_Files.Gnatcov_Prefix = Support_Files.Not_Found then
+      Warning_Or_Error
+        ("Could not locate the invoked gnatcov command: "
+	 & (if Command_Name /= "" then Command_Name else "<empty string>")
+	 & ". If gnatcov is installed on a case sensitive filesystem or"
+	 & " directory, ensure the casing as the executable filename is"
+	 & " used when invoking the program.");
+      Set_Exit_Status (Failure);
+      raise Xcov_Exit_Exc;
+   end if;
+
    --  Parse arguments just enough to determine the target, then run the
    --  corresponding bits-specific main (see gnatcov_arch_specific.adb),
    --  forwarding all arguments as-is.

tools/gnatcov/support_files.ads

@@ -23,6 +23,7 @@ with Ada.Environment_Variables; use Ada.Environment_Variables;
 with GNAT.OS_Lib;
 
 package Support_Files is
+   use type GNAT.OS_Lib.String_Access;
 
    --  This package offers various helpers to locate support files in gnatcov's
    --  installation prefix.
@@ -35,18 +36,30 @@ package Support_Files is
    --  Name of the environment variable which, when defined, provides the name
    --  of the "gnatcov" command that the user ran.
 
+   Not_Found : constant String := "GNATCOV_NOT_FOUND";
+   --  Magic value propagated throughout the elaboration if we can't lookup
+   --  gnatcov on PATH. An error will be reported after elaboration completes.
+
    Gnatcov_Command_Name : String :=
      (if Value (Command_Name_Envvar, "") = ""
       then Command_Name
       else Value (Command_Name_Envvar));
 
-   Gnatcov_Dir : constant String := Containing_Directory
-     (GNAT.OS_Lib.Locate_Exec_On_Path (Command_Name).all);
-   --  Directory that contains the current program
+   Gnatcov_Exec : constant GNAT.OS_Lib.String_Access :=
+     GNAT.OS_Lib.Locate_Exec_On_Path (Command_Name);
+
+   Gnatcov_Dir : constant String :=
+     (if Gnatcov_Exec /= null
+      then Containing_Directory (Gnatcov_Exec.all)
+      else Not_Found);
+   --  Use a magic value when we can't locate gnatcov on path
 
    Gnatcov_Prefix : constant String :=
       (if Value (Prefix_Envvar, "") = ""
-       then Containing_Directory (Gnatcov_Dir)
+       then
+         (if Gnatcov_Dir /= Not_Found
+          then Containing_Directory (Gnatcov_Dir)
+          else Not_Found)
        else Value (Prefix_Envvar));
    --  Installation prefix for gnatcov