Deactivate previous env before activating new one (#25409)

* Deactivate previous env before activating new one

Currently on develop you can run `spack env activate` multiple times to switch
between environments, but they leave traces, even though Spack only supports
one active environment at a time.

Currently:

```console
$ spack env create a
$ spack env create b
$ spack env activate -p a
[a] $ spack env activate -p b
[b] [a] $ spack env activate -p b
[a] [b] [a] $ spack env activate -p a
[a] [b] [c] $ echo $MANPATH | tr ":" "\n"
/path/to/environments/a/.spack-env/view/share/man
/path/to/environments/a/.spack-env/view/man
/path/to/environments/b/.spack-env/view/share/man
/path/to/environments/b/.spack-env/view/man
```

This PR fixes that:

```console
$ spack env activate -p a
[a] $ spack env activate -p b
[b] $ spack env activate -p a
[a] $ echo $MANPATH | tr ":" "\n"
/path/to/environments/a/.spack-env/view/share/man
/path/to/environments/a/.spack-env/view/man
```

1 files changed, 46 insertions, 4 deletions

diff --git a/share/spack/qa/setup-env-test.fish b/share/spack/qa/setup-env-test.fish
index 5cd257aa86..c48ecf4bc4 100755
--- a/share/spack/qa/setup-env-test.fish
+++ b/share/spack/qa/setup-env-test.fish
@@ -88,7 +88,7 @@ end
 function spt_succeeds
     printf "'$argv' succeeds ... "
 
-    set -l output (eval $argv 2>&1)
+    set -l output ($argv 2>&1)
 
     # Save the command result
     set cmd_status $status
@@ -115,7 +115,7 @@ end
 function spt_fails
     printf "'$argv' fails ... "
 
-    set -l output (eval $argv 2>&1)
+    set -l output ($argv 2>&1)
 
     if test $status -eq 0
         fail
@@ -143,7 +143,7 @@ function spt_contains
 
     printf "'$remaining_args' output contains '$target_string' ... "
 
-    set -l output (eval $remaining_args 2>&1)
+    set -l output ($remaining_args 2>&1)
 
     # Save the command result
     set cmd_status $status
@@ -167,6 +167,41 @@ end
 
 
 #
+# Ensure that a string is not in the output of a command. The command must have a 0 exit
+# status to guard against false positives. Suppresses output on success.
+# On failure, echo the exit code and output.
+#
+function spt_does_not_contain
+    set -l target_string $argv[1]
+    set -l remaining_args $argv[2..-1]
+
+    printf "'$remaining_args' does not contain '$target_string' ... "
+
+    set -l output ($remaining_args 2>&1)
+
+    # Save the command result
+    set cmd_status $status
+
+    if test $cmd_status -ne 0
+        fail
+        echo_red "Command exited with error $cmd_status."
+    else if not echo "$output" | string match -q -r ".*$target_string.*"
+        pass
+        return
+    else
+        fail
+        echo_red "'$target_string' was in the output."
+    end
+    if test -n "$output"
+        echo_msg "Output:"
+        echo "$output"
+    else
+        echo_msg "No output."
+    end
+end
+
+
+#
 # Ensure that a variable is set.
 #
 function is_set
@@ -255,12 +290,13 @@ spack -m install --fake a
 # create a test environment for testing environment commands
 echo "Creating a mock environment"
 spack env create spack_test_env
+spack env create spack_test_2_env
 
 # ensure that we uninstall b on exit
 function spt_cleanup -p %self
     echo "Removing test environment before exiting."
     spack env deactivate 2>&1 > /dev/null
-    spack env rm -y spack_test_env
+    spack env rm -y spack_test_env spack_test_2_env
 
     title "Cleanup"
     echo "Removing test packages before exiting."
@@ -366,6 +402,12 @@ is_set SPACK_ENV
 spack env deactivate
 is_not_set SPACK_ENV
 
+echo "Testing spack env activate repeatedly"
+spack env activate spack_test_env
+spack env activate spack_test_2_env
+spt_contains 'spack_test_2_env' 'fish' '-c' 'echo $PATH'
+spt_does_not_contain 'spack_test_env' 'fish' '-c' 'echo $PATH'
+
 #
 # NOTE: `--prompt` on fish does nothing => currently not implemented.
 #