[PATCH v2 2/2] builtins: kill: Do not signal current process group when pid is empty

[Chris Down <chris@chrisdown.name>]

The following case was encountered in the wild:

    % zsh; echo "$?"
    % trap 'exit 5' TERM
    % kill ''
    5

This behaviour seems more likely to be the result of bugs in programs
(eg. `kill -9 "$unsetvar") rather than being desirable behaviour to me.
It also seems unintentional judging by the code and documentation, since
it comes about as a result of the fact that:

- `isanum` returns true for empty strings (since an empty string
  technically only consists of digits and minuses...);
- `atoi`, when passed a pointer to an invalid number, returns 0;
- `kill(0, signal)` sends the signal in question to all processes in the
  current process group.

There are (at least) two ways to solve this issue:

1. Add special handling to `kill` to avoid this case. See this patch[0]
   for a version that does that.
2. Change how isanum behaves. Since the only two call sites that use it
   both seem like they should handle the case where the input char array
   is empty, that seems like a reasonable overall change to me.[1]

After this patch:

    % trap 'exit 5' TERM
    % kill ''
    kill: illegal pid:

0: https://www.zsh.org/mla/workers/2020/msg00251.html
1: The other callsite using isanum() is the fg builtin, but in that case
   we just fail later since we can't find any job named '', so no
   big deal either way. It's the kill case which is more concerning.
---
 Src/jobs.c        |  5 +++--
 Test/B11kill.ztst | 12 ++++++++++++
 2 files changed, 15 insertions(+), 2 deletions(-)

diff --git a/Src/jobs.c b/Src/jobs.c
index e7438251e..0485f2c7c 100644
--- a/Src/jobs.c
+++ b/Src/jobs.c
@@ -1854,13 +1854,14 @@ scanjobs(void)
 
 /* This simple function indicates whether or not s may represent      *
  * a number.  It returns true iff s consists purely of digits and     *
- * minuses.  Note that minus may appear more than once, and the empty *
- * string will produce a `true' response.                             */
+ * minuses.  Note that minus may appear more than once.               */
 
 /**/
 static int
 isanum(char *s)
 {
+    if (*s == '\0')
+	return 0;
     while (*s == '-' || idigit(*s))
 	s++;
     return *s == '\0';
diff --git a/Test/B11kill.ztst b/Test/B11kill.ztst
index 6a548213b..69d4388c7 100644
--- a/Test/B11kill.ztst
+++ b/Test/B11kill.ztst
@@ -50,3 +50,15 @@
   kill -INT
 1:kill with sigspec only
 ?(eval):kill:1: not enough arguments
+
+# Regression tests: `kill ''` should not result in `kill 0`.
+
+  trap 'exit 19' TERM
+  kill ''
+1:Plain kill with empty pid should not send signal to current process group
+?(eval):kill:2: illegal pid: 
+
+  trap 'exit 11' INT
+  kill -INT ''
+1:kill with empty pid and sigspec should not send signal to current process group
+?(eval):kill:2: illegal pid: 
-- 
2.25.0