Fix float64 comparison test failure on archs using FMA (#1133)

* Fix float64 comparison test failure on archs using FMA

Architectures using FMA optimization yield slightly different results so
we cannot assume floating point values will be precisely the same across
different architectures.

The solution in this change is to check "abs(a-b) < tolerance" instead
of comparing the exact values. This will give us confidence that the
histogram buckets are near identical.

Signed-off-by: Seth Bunce <[email protected]>

* Apply suggestions from code review

Co-authored-by: Daniel Swarbrick <[email protected]>
Signed-off-by: Seth Bunce <[email protected]>

* copy float compare dependency

Per discussion in the pull request, we'd like to avoid having an extra
dependency on a float comparison package. Instead, we copy the float compare
functions from the float comparison package.

The float comparison package we're choosing is this. The author of this
package has commented in the pull request and it looks like we have consensus
that this is the best option.
github.com/beorn7/floats

Signed-off-by: Seth Bunce <[email protected]>

* remove float32 variant, relocate into separate file

This change removes the float32 variant of the AlmostEqual funcs, that we will
likely never use. This change also relocates the function into a separate file
to avoid modifying a file that's a fork of another vendored package.

Signed-off-by: Seth Bunce <[email protected]>

Signed-off-by: Seth Bunce <[email protected]>
Signed-off-by: Seth Bunce <[email protected]>
Co-authored-by: Daniel Swarbrick <[email protected]>

prometheus/histogram_test.go

@@ -29,6 +29,8 @@ import (
 	"github.com/golang/protobuf/proto"
 	"google.golang.org/protobuf/types/known/timestamppb"
 
+	"github.com/prometheus/client_golang/prometheus/internal"
+
 	dto "github.com/prometheus/client_model/go"
 )
 
@@ -356,13 +358,12 @@ func TestBuckets(t *testing.T) {
 
 	got = ExponentialBucketsRange(1, 100, 10)
 	want = []float64{
-		1.0, 1.6681005372000588, 2.782559402207125,
-		4.641588833612779, 7.742636826811273, 12.915496650148842,
-		21.544346900318846, 35.93813663804629, 59.94842503189414,
-		100.00000000000007,
+		1.0, 1.6681, 2.7825, 4.6415, 7.7426, 12.9154, 21.5443,
+		35.9381, 59.9484, 100.0000,
 	}
-	if !reflect.DeepEqual(got, want) {
-		t.Errorf("exponential buckets range: got %v, want %v", got, want)
+	const epsilon = 0.0001
+	if !internal.AlmostEqualFloat64s(got, want, epsilon) {
+		t.Errorf("exponential buckets range: got %v, want %v (epsilon %f)", got, want, epsilon)
 	}
 }
 

prometheus/internal/almost_equal.go

@@ -0,0 +1,60 @@
+// Copyright (c) 2015 Björn Rabenstein
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to deal
+// in the Software without restriction, including without limitation the rights
+// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in all
+// copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+// SOFTWARE.
+//
+// The code in this package is copy/paste to avoid a dependency. Hence this file
+// carries the copyright of the original repo.
+// https://github.com/beorn7/floats
+package internal
+
+import (
+	"math"
+)
+
+// minNormalFloat64 is the smallest positive normal value of type float64.
+var minNormalFloat64 = math.Float64frombits(0x0010000000000000)
+
+// AlmostEqualFloat64 returns true if a and b are equal within a relative error
+// of epsilon. See http://floating-point-gui.de/errors/comparison/ for the
+// details of the applied method.
+func AlmostEqualFloat64(a, b, epsilon float64) bool {
+	if a == b {
+		return true
+	}
+	absA := math.Abs(a)
+	absB := math.Abs(b)
+	diff := math.Abs(a - b)
+	if a == 0 || b == 0 || absA+absB < minNormalFloat64 {
+		return diff < epsilon*minNormalFloat64
+	}
+	return diff/math.Min(absA+absB, math.MaxFloat64) < epsilon
+}
+
+// AlmostEqualFloat64s is the slice form of AlmostEqualFloat64.
+func AlmostEqualFloat64s(a, b []float64, epsilon float64) bool {
+	if len(a) != len(b) {
+		return false
+	}
+	for i := range a {
+		if !AlmostEqualFloat64(a[i], b[i], epsilon) {
+			return false
+		}
+	}
+	return true
+}