fix: handle incompatible schemas in histogram aggregation

When downsampling histograms, if the schema type changes between
custom bucket histograms (schema < 0) and exponential bucket histograms
(schema >= 0), the Add operation would fail because these histogram
types cannot be added together.

This fix checks for incompatible schemas before attempting to add
histograms. When incompatible schemas are detected, the aggregation
is reset to the new histogram, similar to how counter resets are handled.

Fixes panic when mixing custom bucket and exponential bucket histograms
during downsampling.

Signed-off-by: Br1an67 <932039080@qq.com>

Author: Br1an67

pkg/compact/downsample/downsample.go

@@ -367,7 +367,14 @@ func (h *histogramAggregator) add(s sample) {
 	}
 
 	if h.total > 0 {
-		if fh.CounterResetHint != histogram.GaugeType && oFh.DetectReset(h.previous) {
+		// Check if schemas are incompatible (custom bucket vs exponential bucket).
+		// Custom bucket histograms (schema < 0) and exponential bucket histograms (schema >= 0)
+		// cannot be added together. Treat as a counter reset.
+		incompatibleSchemas := (h.counter.Schema < 0) != (fh.Schema < 0)
+		if incompatibleSchemas {
+			// Schema type changed, treat as counter reset.
+			h.counter = fh.Copy()
+		} else if fh.CounterResetHint != histogram.GaugeType && oFh.DetectReset(h.previous) {
 			// Counter reset, correct the value.
 			mustHistogramOp(h.counter.Add(fh))
 		} else if oFh.Schema < 0 {
@@ -391,7 +398,15 @@ func (h *histogramAggregator) add(s sample) {
 	if h.sum == nil {
 		h.sum = fh.Copy()
 	} else {
-		mustHistogramOp(h.sum.Add(fh))
+		// Check if schemas are incompatible (custom bucket vs exponential bucket).
+		// Custom bucket histograms (schema < 0) and exponential bucket histograms (schema >= 0)
+		// cannot be added together. Reset the sum to the new histogram.
+		incompatibleSchemas := (h.sum.Schema < 0) != (fh.Schema < 0)
+		if incompatibleSchemas {
+			h.sum = fh.Copy()
+		} else {
+			mustHistogramOp(h.sum.Add(fh))
+		}
 	}
 
 	// This needs to be h gauge histogram, otherwise reset detection will be triggered