Require a main program (fix jqlang#2785)

docs/content/manual/manual.yml

@@ -82,9 +82,18 @@ sections:
       output(s) of the filter are written to standard output, as a
       sequence of newline-separated JSON data.
 
+      The simplest and most common filter (or jq program) is `.`,
+      which is the identity operator, copying the inputs of the jq
+      processor to the output stream.  Because the default behavior of
+      the jq processor is to read JSON texts from the input stream,
+      and to pretty-print outputs, the `.` program's main use is to
+      validate and pretty-print the inputs.  The jq programming
+      language is quite rich and allows for much more than just
+      validation and pretty-printing.
+
       Note: it is important to mind the shell's quoting rules.  As a
       general rule it's best to always quote (with single-quote
-      characters) the jq program, as too many characters with special
+      characters on Unix shells) the jq program, as too many characters with special
       meaning to jq are also shell meta-characters.  For example, `jq
       "foo"` will fail on most Unix shells because that will be the same
       as `jq foo`, which will generally fail because `foo is not
@@ -101,6 +110,9 @@ sections:
       * Powershell: `jq '.[\"foo\"]'`
       * Windows command shell: `jq ".[\"foo\"]"`
 
+      Note: jq allows user-defined functions, but every jq program
+      must have a top-level expression.
+
       You can affect how jq reads and writes its input and output
       using some command-line options:
 

src/linker.c

@@ -407,6 +407,12 @@ int load_program(jq_state *jq, struct locfile* src, block *out_block) {
   if (nerrors)
     return nerrors;
 
+  if (!block_has_main(program)) {
+    jq_report_error(jq, jv_string_fmt("jq: error: Top-level program given (try \".\")\n"));
+    block_free(program);
+    return 1;
+  }
+
   char* home = getenv("HOME");
   if (home) {    // silently ignore no $HOME
     /* Import ~/.jq as a library named "" found in $HOME */

tests/no-main-program.jq

@@ -0,0 +1 @@
+def a: .;

tests/shtest

@@ -496,4 +496,34 @@ if command -v script >/dev/null 2>&1; then
   cmp $d/color $d/expect
 fi
 
+# #2785
+if $VALGRIND $Q $JQ -n -f "$JQTESTDIR/no-main-program.jq" > $d/out 2>&1; then
+  echo "jq -f $JQTESTDIR/no-main-program.jq succeeded"
+  exit 1
+else
+  EC=$?
+  if [ $EC -eq 1 ]; then
+    echo "jq -f $JQTESTDIR/no-main-program.jq faile with memory errors"
+    exit 1
+  fi
+  if [ $EC -ne 3 ]; then
+    echo "jq -f $JQTESTDIR/no-main-program.jq faile with wrong exit code ($EC)"
+    exit 1
+  fi
+fi
+cat > $d/expected <<EOF
+jq: error: Top-level program given (try ".")
+
+jq: 1 compile error
+EOF
+if ! diff $d/expected $d/out; then
+  echo "jq -f $JQTESTDIR/no-main-program.jq failed but its error message is not the expected one"
+  exit 1
+fi
+
+if ! $VALGRIND $Q $JQ -n -f "$JQTESTDIR/yes-main-program.jq" > $d/out 2>&1; then
+  echo "jq -f $JQTESTDIR/yes-main-program.jq failed"
+  exit 1
+fi
+
 exit 0

tests/yes-main-program.jq

@@ -0,0 +1,2 @@
+def a: .;
+0