Add filename/line functions to jq (fix #753)

This adds `input_filename` and `input_line_number` built-in functions
for use in, for example, `error` messages.

Example:

    $ printf '{"a":1}\n{"a":2}\n' > 4.json
    $ printf '{"a":"hello"}\n' > 5.json
    $ ./jq '{ "file":input_filename, "line":input_line_number, "value":.a }' 4.json 5.json
    {
      "file": "4.json",
      "line": 1,
      "value": 1
    }
    {
      "file": "4.json",
      "line": 2,
      "value": 2
    }
    {
      "file": "5.json",
      "line": 1,
      "value": "hello"
    }

5 files changed, 53 insertions, 0 deletions

diff --git a/builtin.c b/builtin.c
index c022416a..4747f618 100644
--- a/builtin.c
+++ b/builtin.c
@@ -1126,6 +1126,12 @@ static jv f_now(jq_state *jq, jv a) {
 }
 #endif
 
+static jv f_current_filename(jq_state *jq) {
+  return jq_util_input_get_current_filename(jq);
+}
+static jv f_current_line(jq_state *jq) {
+  return jq_util_input_get_current_line(jq);
+}
 
 #define LIBM_DD(name) \
   {(cfunction_ptr)f_ ## name, "_" #name, 1},
@@ -1188,6 +1194,8 @@ static const struct cfunction function_list[] = {
   {(cfunction_ptr)f_mktime, "mktime", 1},
   {(cfunction_ptr)f_gmtime, "gmtime", 1},
   {(cfunction_ptr)f_now, "now", 1},
+  {(cfunction_ptr)f_current_filename, "input_filename", 1},
+  {(cfunction_ptr)f_current_line, "input_line_number", 1},
 };
 #undef LIBM_DD
 
diff --git a/docs/content/3.manual/manual.yml b/docs/content/3.manual/manual.yml
index 39f69fe1..d55df30f 100644
--- a/docs/content/3.manual/manual.yml
+++ b/docs/content/3.manual/manual.yml
@@ -2413,6 +2413,17 @@ sections:
           `["DEBUG:", <input-value>]` and prints that and a newline on
           stderr, compactly.  This may change in the future.
 
+      - title: "`input_filename`"
+        body: |
+
+          Returns the name of the file whose input is currently being
+          filtered.
+
+      - title: "`input_line_number`"
+        body: |
+
+          Returns the line number of the input currently being filtered.
+
   - title: Assignment
     body: |
 
diff --git a/jq.h b/jq.h
index 99e248ab..2c5ef168 100644
--- a/jq.h
+++ b/jq.h
@@ -47,5 +47,7 @@ int jq_util_input_errors(jq_util_input_state);
 jv jq_util_input_next_input(jq_util_input_state);
 jv jq_util_input_next_input_cb(jq_state *, void *);
 jv jq_util_input_get_position(jq_state*);
+jv jq_util_input_get_current_filename(jq_state*);
+jv jq_util_input_get_current_line(jq_state*);
 
 #endif /* !JQ_H */
diff --git a/tests/all.test b/tests/all.test
index 106afaa4..5a87398b 100644
--- a/tests/all.test
+++ b/tests/all.test
@@ -1173,3 +1173,12 @@ import "test_bind_order" as check; check::check
 null
 true
 
+try input_filename catch .
+null
+"Unknown input filename"
+
+try input_line_number catch .
+null
+"Unknown input line number"
+
+
diff --git a/util.c b/util.c
index f6033b73..c8dd4f6d 100644
--- a/util.c
+++ b/util.c
@@ -341,6 +341,29 @@ jv jq_util_input_get_position(jq_state *jq) {
   return v;
 }
 
+jv jq_util_input_get_current_filename(jq_state* jq) {
+  jq_input_cb cb=NULL;
+  void *cb_data=NULL;
+  jq_get_input_cb(jq, &cb, &cb_data);
+  if (cb != jq_util_input_next_input_cb)
+    return jv_invalid_with_msg(jv_string("Unknown input filename"));
+  jq_util_input_state s = (jq_util_input_state)cb_data;
+  jv v = jv_copy(s->current_filename);
+  return v;
+}
+
+jv jq_util_input_get_current_line(jq_state* jq) {
+  jq_input_cb cb=NULL;
+  void *cb_data=NULL;
+  jq_get_input_cb(jq, &cb, &cb_data);
+  if (cb != jq_util_input_next_input_cb)
+    return jv_invalid_with_msg(jv_string("Unknown input line number"));
+  jq_util_input_state s = (jq_util_input_state)cb_data;
+  jv v = jv_number(s->current_line);
+  return v;
+}
+
+
 // Blocks to read one more input from stdin and/or given files
 // When slurping, it returns just one value
 jv jq_util_input_next_input(jq_util_input_state state) {