Fix PR bpftrace#1416: unsigned int overflow bug and unstable tests

The merged PR which enables `top` and `div` args of `print` for `avg` maps has
two issues that need addressing:

1. It came with subtractions of unsigned integers which can easily cause
negative overflows.
2. The runtime tests it came with are somehow unstable and fails randomly. This
patch brings more straightforward tests.

CHANGELOG.md

@@ -28,6 +28,8 @@ and this project adheres to
 #### Removed
 
 #### Fixed
+- Fix negative overflow bug and unstable tests in PR #1416
+  - [#1416](https://github.com/iovisor/bpftrace/pull/1436)
 
 #### Tools
 

src/output.cpp

@@ -290,11 +290,9 @@ void TextOutput::map_stats(
     auto &key = key_count.first;
     auto &value = values_by_key.at(key);
 
-    if (map.type_.IsAvgTy() && top)
-    {
-      if (i++ < (values_by_key.size() - top))
-        continue;
-    }
+    if (map.type_.IsAvgTy() && top && values_by_key.size() > top &&
+        i++ < (values_by_key.size() - top))
+      continue;
 
     out_ << map.name_ << map.key_.argument_value_list_str(bpftrace, key) << ": ";
 
@@ -610,11 +608,9 @@ void JsonOutput::map_stats(
     auto &key = key_count.first;
     auto &value = values_by_key.at(key);
 
-    if (map.type_.IsAvgTy() && top)
-    {
-      if (j++ < (values_by_key.size() - top))
-        continue;
-    }
+    if (map.type_.IsAvgTy() && top && values_by_key.size() > top &&
+        j++ < (values_by_key.size() - top))
+      continue;
 
     std::vector<std::string> args = map.key_.argument_value_list(bpftrace, key);
     if (i > 0)

tests/runtime/call

@@ -216,12 +216,12 @@ RUN bpftrace -v -e 'BEGIN { @[nsecs] = strftime("%m/%d/%y", nsecs); exit();}'
 EXPECT @\[[0-9]*\]: [0-9]{2}\/[0-9]{2}\/[0-9]{2}
 TIMEOUT 5
 
-NAME print_avg_map_top
-RUN bpftrace -e 'i:ms:1 { @[nsecs/100000] = avg(1) } i:ms:10 { print(@, 3, 0); exit(); } END {clear(@)}' | grep -c -v "^ *$"
-EXPECT ^4$
+NAME print_avg_map_args
+RUN bpftrace -e 'BEGIN { @["a"] = avg(10); @["b"] = avg(20); @["c"] = avg(30); @["d"] = avg(40); print(@, 2, 10); print("END"); clear(@); exit(); }'
+EXPECT Attaching 1 probe\.\.\.\n@\[c\]: 3\n@\[d\]: 4\n\nEND
 TIMEOUT 1
 
-NAME print_avg_map_div
-RUN bpftrace -e 'i:ms:1 { @[nsecs/1000000] = avg(10) } i:ms:10 { print(@, 1, 10) ;exit();} END {clear(@)}'
-EXPECT ^@\[[0-9]+\]: 1$
+NAME print_avg_map_with_large_top
+RUN bpftrace -e 'BEGIN { @["a"] = avg(10); @["b"] = avg(20); @["c"] = avg(30); @["d"] = avg(40); print(@, 10, 10); print("END"); clear(@); exit(); }'
+EXPECT Attaching 1 probe\.\.\.\n@\[a\]: 1\n@\[b\]: 2\n@\[c\]: 3\n@\[d\]: 4\n\nEND
 TIMEOUT 1

tests/runtime/json-output

@@ -106,6 +106,11 @@ EXPECT ^{"type": "value", "data": \[1,2,"string"\]}$
 TIMEOUT 1
 
 NAME print_avg_map_args
-RUN bpftrace -f json -e 'i:ms:1 { @[nsecs/100000] = avg(10) } i:ms:10 { print(@, 1, 10); exit(); } END {clear(@)}'
-EXPECT ^{"type": "stats", "data": {"@": { *"[0-9]*": 1}}}$
-TIMEOUT 1
+RUN bpftrace -f json -e 'BEGIN { @["a"] = avg(10); @["b"] = avg(20); @["c"] = avg(30); @["d"] = avg(40); print(@, 2, 10); clear(@); exit(); }'
+EXPECT {"type": "stats", "data": {"@": { *"c": 3, *"d": 4}}}
+TIMEOUT 1
+
+NAME print_avg_map_with_large_top
+RUN bpftrace -f json -e 'BEGIN { @["a"] = avg(10); @["b"] = avg(20); @["c"] = avg(30); @["d"] = avg(40); print(@, 10, 10); clear(@); exit(); }'
+EXPECT {"type": "stats", "data": {"@": { *"a": 1, *"b": 2, *"c": 3, *"d": 4}}}
+TIMEOUT 1