Memory usage in CG Slice path could be 0 (#6110)

* Memory usage in Slice path could be 0

* Update memory usage test to handle zero values and refine method name

* Update error message for memory usage exception to clarify non-negative expectation

* Merge test cases

Author: makazeu

src/Libraries/Microsoft.Extensions.Diagnostics.ResourceMonitoring/Linux/LinuxUtilizationParserCgroupV2.cs

@@ -257,11 +257,11 @@ public long GetMemoryUsageInBytesFromSlices(string pattern)
             ReadOnlySpan<char> memoryUsageFile = bufferWriter.Buffer.WrittenSpan;
             int next = GetNextNumber(memoryUsageFile, out long containerMemoryUsage);
 
-            if (containerMemoryUsage == 0 || containerMemoryUsage == -1)
+            if (containerMemoryUsage == -1)
             {
                 memoryUsageInBytesTotal = 0;
                 Throw.InvalidOperationException(
-                    $"We tried to read '{memoryUsageInBytesFile}', and we expected to get a positive number but instead it was: '{memoryUsageFile}'.");
+                    $"We tried to read '{memoryUsageInBytesFile}', and we expected to get a non-negative number but instead it was: '{memoryUsageFile}'.");
             }
 
             memoryUsageInBytesTotal += containerMemoryUsage;

test/Libraries/Microsoft.Extensions.Diagnostics.ResourceMonitoring.Tests/Linux/LinuxUtilizationParserCgroupV2Tests.cs

@@ -168,6 +168,7 @@ public Task Throws_When_UsageInBytes_Doesnt_Contain_A_Number()
     [ConditionalFact]
     public void Returns_Memory_Usage_When_Memory_Usage_Is_Valid()
     {
+        // When memory usage is a positive number
         var regexPatternforSlices = @"\w+.slice";
         var f = new HardcodedValueFileSystem(new Dictionary<FileInfo, string>
         {
@@ -178,6 +179,16 @@ public void Returns_Memory_Usage_When_Memory_Usage_Is_Valid()
         var r = p.GetMemoryUsageInBytesFromSlices(regexPatternforSlices);
 
         Assert.Equal(5_342_342, r);
+
+        // When memory usage is zero
+        f = new HardcodedValueFileSystem(new Dictionary<FileInfo, string>
+        {
+            { new FileInfo("/sys/fs/cgroup/system.slice/memory.current"), "0"},
+        });
+
+        p = new LinuxUtilizationParserCgroupV2(f, new FakeUserHz(100));
+        r = p.GetMemoryUsageInBytesFromSlices(regexPatternforSlices);
+        Assert.Equal(0, r);
     }
 
     [ConditionalTheory]

test/Libraries/Microsoft.Extensions.Diagnostics.ResourceMonitoring.Tests/Linux/Verified/LinuxUtilizationParserCgroupV2Tests.Throws_When_UsageInBytes_Doesnt_Contain_A_Number.verified.txt

@@ -1,6 +1,6 @@
 {
   Type: InvalidOperationException,
-  Message: We tried to read '/sys/fs/cgroup/system.slice/memory.current', and we expected to get a positive number but instead it was: 'dasda'.,
+  Message: We tried to read '/sys/fs/cgroup/system.slice/memory.current', and we expected to get a non-negative number but instead it was: 'dasda'.,
   StackTrace:
 at Microsoft.Shared.Diagnostics.Throw.InvalidOperationException(String message)
 at Microsoft.Extensions.Diagnostics.ResourceMonitoring.Linux.LinuxUtilizationParserCgroupV2.GetMemoryUsageInBytesFromSlices(String pattern)