Make desched_sigkill test handle the case where the phase-1 read() …

…does not fill `chs` in one call

Currently `chs` is not fully initialized in that case, which causes failures because the uninitialized bytes are written to `parent_to_child` and the children check they're equal to `x`.

Resolves #3559

src/test/desched_sigkill.c

@@ -48,11 +48,14 @@ int main(void) {
 
  // Phase 1: Wait for all readers to become ready.
  char chs[NUM_READERS];
- int sum = 0;
- while (sum < NUM_READERS) {
- ret = read(child_to_parent[0], &chs, NUM_READERS);
+ int bytes_read = 0;
+ while (bytes_read < NUM_READERS) {
+ ret = read(child_to_parent[0], &chs[bytes_read], NUM_READERS - bytes_read);
  test_assert(ret > 0);
- sum += ret;
+ for (int i = 0; i < ret; ++i) {
+ test_assert(chs[bytes_read + i] == 'x');
+ }
+ bytes_read += ret;
  }
 
  // Phase 2: Release readers from `read` syscall.