define the var when the read limit is exceeded

This problem was found when testing the change for issue #4163 to make
dereferencing an undefined variable an error.

Fixes #4180

src/builtin_read.cpp

@@ -396,6 +396,9 @@ int builtin_read(parser_t &parser, io_streams_t &streams, wchar_t **argv) {
     }
 
     if (optind == argc || exit_res != STATUS_CMD_OK) {
+        // Define the var without any data. We do this because when this happens we want the user to
+        // be able to use the var but have it expand to nothing.
+        env_set(argv[optind], NULL, opts.place);
         return exit_res;
     }
 

tests/read.in

@@ -141,9 +141,12 @@ yes $line | dd bs=1024 count=(math "1 + $FISH_READ_BYTE_LIMIT / 1024") ^/dev/nul
 if test $status -ne 122
     echo reading too much data did not terminate with failure status
 end
-if test (string length "$x") -ne 0
-    echo reading too much data resulted in a var with unexpected data
-end
+# The read var should be defined but not have any elements when the read
+# aborts due to too much data.
+set -q x
+or echo reading too much data did not define the var
+set -q x[1]
+and echo reading too much data resulted in a var with unexpected data
 
 # Ensure the `read` command terminates if asked to read too much data even if
 # given an explicit limit. The var should be empty. We throw away any data we
@@ -152,9 +155,10 @@ yes $line | read --null --nchars=(math "$FISH_READ_BYTE_LIMIT + 1") x
 if test $status -ne 122
     echo reading too much data did not terminate with failure status
 end
-if test (string length "$x") -ne 0
-    echo reading too much data resulted in a var with unexpected data
-end
+set -q x
+or echo reading too much data with --nchars did not define the var
+set -q x[1]
+and echo reading too much data with --nchars resulted in a var with unexpected data
 
 # Now do the opposite of the previous test and confirm we can read reasonable
 # amounts of data.