Handle linear histograms with a single #0 value.

Some enumerations have only a single entry of 0 (which is technically
an "underflow").  Support this by changing the maximum from 1 to 2.

Bug: 904964
Change-Id: If81522c55da292948f6a04cc51077111200537f1
Reviewed-on: https://chromium-review.googlesource.com/c/1336208Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Commit-Queue: Brian White <bcwhite@chromium.org>
Cr-Commit-Position: refs/heads/master@{#608493}

base/metrics/histogram.cc

@@ -1026,6 +1026,18 @@ HistogramBase* LinearHistogram::FactoryGetWithRangeDescription(
     uint32_t bucket_count,
     int32_t flags,
     const DescriptionPair descriptions[]) {
+  // Originally, histograms were required to have at least one sample value
+  // plus underflow and overflow buckets. For single-entry enumerations,
+  // that one value is usually zero (which IS the underflow bucket)
+  // resulting in a |maximum| value of 1 (the exclusive upper-bound) and only
+  // the two outlier buckets. Handle this by making max==2 and buckets==3.
+  // This usually won't have any cost since the single-value-optimization
+  // will be used until the count exceeds 16 bits.
+  if (maximum == 1 && bucket_count == 2) {
+    maximum = 2;
+    bucket_count = 3;
+  }
+
   bool valid_arguments = Histogram::InspectConstructionArguments(
       name, &minimum, &maximum, &bucket_count);
   DCHECK(valid_arguments);

base/metrics/histogram_unittest.cc

@@ -295,6 +295,25 @@ TEST_P(HistogramTest, LinearRangesTest) {
   EXPECT_TRUE(ranges2.Equals(histogram2->bucket_ranges()));
 }
 
+TEST_P(HistogramTest, SingleValueEnumerationHistogram) {
+  // Make sure its possible to construct a linear histogram with only the two
+  // required outlier buckets (underflow and overflow).
+  HistogramBase* histogram = LinearHistogram::FactoryGet(
+      "SingleValueEnum", 1, 1, 2, HistogramBase::kNoFlags);
+  EXPECT_TRUE(histogram);
+
+  // Make sure the macros work properly. This can only be run when
+  // there is no persistent allocator which can be discarded and leave
+  // dangling pointers.
+  if (!use_persistent_histogram_allocator_) {
+    enum EnumWithMax {
+      kSomething = 0,
+      kMaxValue = kSomething,
+    };
+    UMA_HISTOGRAM_ENUMERATION("h1", kSomething);
+  }
+}
+
 TEST_P(HistogramTest, ArrayToCustomEnumRangesTest) {
   const HistogramBase::Sample ranges[3] = {5, 10, 20};
   std::vector<HistogramBase::Sample> ranges_vec =