bmalloc: check WPE_RAM_SIZE

WPE_RAM_SIZE env can be used to control the reported available
total ram (WTF/wtf/RAMSize.cpp). Bmalloc is unaware of
this setting, leading to discrepancy between the current
memory pressure status as seen by bmalloc & reset of wpe, which makes
bmalloc uneager to scavenge & return memory to the system on time.

Author: tomasz-karczewski-red

Source/bmalloc/bmalloc/AvailableMemory.cpp

@@ -149,6 +149,29 @@ struct LinuxMemory {
 };
 #endif
 
+static size_t customRAMSize()
+{
+    // Syntax: Case insensitive, unit multipliers (M=Mb, K=Kb, <empty>=bytes).
+    // Example: WPE_RAM_SIZE='500M'
+
+    size_t customSize = 0;
+
+    const char *s = getenv("WPE_RAM_SIZE");
+    if (s) {
+        size_t len = strlen(s);
+        size_t val = atoi(s);
+        size_t units = 1;
+        if (s[len-1] == 'k' || s[len-1] == 'K')
+            units = 1024;
+        else if (s[len-1] == 'm' || s[len-1] == 'M')
+            units = 1024*1024;
+        return units * val;
+    }
+
+    return customSize;
+}
+
+
 static size_t computeAvailableMemory()
 {
 #if BOS(DARWIN)
@@ -162,6 +185,10 @@ static size_t computeAvailableMemory()
     // (for example) and we have code that depends on those boundaries.
     return ((sizeAccordingToKernel + multiple - 1) / multiple) * multiple;
 #elif BOS(FREEBSD) || BOS(LINUX)
+    size_t customRamSize = customRAMSize();
+    if (customRamSize) {
+        return customRamSize;
+    }
     struct sysinfo info;
     if (!sysinfo(&info))
         return info.totalram * info.mem_unit;