TODO fixed: Detect when called from within callbacks

Closes #2302

tests/data/Makefile.inc

@@ -171,7 +171,7 @@ test1520 test1521 \
 test1525 test1526 test1527 test1528 test1529 test1530 test1531 test1532 \
 test1533 test1534 test1535 test1536 test1537 test1538 \
 test1540 \
-test1550 test1551 test1552 test1553 test1554 \
+test1550 test1551 test1552 test1553 test1554 test1555 \
 test1600 test1601 test1602 test1603 test1604 test1605 test1606 \
 \
 test1700 test1701 test1702 \

tests/data/test1538

@@ -125,7 +125,8 @@ e89: The max connection limit is reached
 e90: SSL public key does not match pinned public key
 e91: SSL server certificate status verification FAILED
 e92: Stream error in the HTTP/2 framing layer
-e93: Unknown error
+e93: API function called from within callback
+e94: Unknown error
 m-1: Please call curl_multi_perform() soon
 m0: No error
 m1: Invalid multi handle
@@ -135,7 +136,8 @@ m4: Internal error
 m5: Invalid socket argument
 m6: Unknown option
 m7: The easy handle is already added to a multi handle
-m8: Unknown error
+m8: API function called from within callback
+m9: Unknown error
 s0: No error
 s1: Unknown share option
 s2: Share currently in use

tests/data/test1555

@@ -0,0 +1,50 @@
+<testcase>
+<info>
+<keywords>
+RECURSIVE_API_CALL
+</keywords>
+</info>
+
+# Server-side
+<reply>
+<data nocheck="yes">
+HTTP/1.1 204 PARTIAL
+X-Comment: partial response to keep the client waiting
+</data>
+<postcmd>
+wait 10
+</postcmd>
+</reply>
+
+# Client-side
+<client>
+<server>
+http
+</server>
+<tool>
+lib1555
+</tool>
+ <name>
+verify api is protected against calls from callbacks
+ </name>
+
+# this server/host won't be used for real
+ <command>
+http://%HOSTIP:%HTTPPORT/1555
+</command>
+</client>
+
+# Verify data after the test has been "shot"
+<verify>
+<protocol>
+</protocol>
+# 42 == CURLE_ABORTED_BY_CALLBACK
+<errorcode>
+42
+</errorcode>
+<stdout>
+curl_easy_recv returned 93
+curl_easy_send returned 93
+</stdout>
+</verify>
+</testcase>