vfs: use geometric doubling for write buffer expansion

Switch MemoryFileHandle from exact-fit allocation to geometric
doubling (capacity * 2) for amortized O(1) append performance.
Track actual content size separately from buffer capacity via
a #size field, and expose only valid data through subarray views.

Author: mcollina

lib/internal/vfs/file_handle.js

@@ -2,6 +2,7 @@
 
 const {
   DateNow,
+  MathMax,
   MathMin,
   Symbol,
 } = primordials;
@@ -253,6 +254,7 @@ class VirtualFileHandle {
  */
 class MemoryFileHandle extends VirtualFileHandle {
   #content;
+  #size;
   #entry;
   #getStats;
 
@@ -273,6 +275,7 @@ class MemoryFileHandle extends VirtualFileHandle {
   constructor(path, flags, mode, content, entry, getStats) {
     super(path, flags, mode);
     this.#content = content;
+    this.#size = content.length;
     this.#entry = entry;
     this.#getStats = getStats;
 
@@ -281,13 +284,14 @@ class MemoryFileHandle extends VirtualFileHandle {
         flags === 'wx' || flags === 'wx+') {
       // Write mode: truncate
       this.#content = Buffer.alloc(0);
+      this.#size = 0;
       if (entry) {
         entry.content = this.#content;
       }
     } else if (flags === 'a' || flags === 'a+' ||
                flags === 'ax' || flags === 'ax+') {
       // Append mode: position at end
-      this.position = this.#content.length;
+      this.position = this.#size;
     }
   }
 
@@ -329,7 +333,7 @@ class MemoryFileHandle extends VirtualFileHandle {
     if (this.#entry?.isDynamic && this.#entry.isDynamic()) {
       return this.#entry.getContentSync();
     }
-    return this.#content;
+    return this.#content.subarray(0, this.#size);
   }
 
   /**
@@ -404,23 +408,30 @@ class MemoryFileHandle extends VirtualFileHandle {
 
     // In append mode, always write at the end
     const writePos = this.#isAppend() ?
-      this.#content.length :
+      this.#size :
       (position !== null && position !== undefined ? position : this.position);
     const data = buffer.subarray(offset, offset + length);
 
-    // Expand content if needed
-    if (writePos + length > this.#content.length) {
-      const newContent = Buffer.alloc(writePos + length);
-      this.#content.copy(newContent, 0, 0, this.#content.length);
+    // Expand buffer if needed (geometric doubling for amortized O(1) appends)
+    const neededSize = writePos + length;
+    if (neededSize > this.#content.length) {
+      const newCapacity = MathMax(neededSize, this.#content.length * 2);
+      const newContent = Buffer.alloc(newCapacity);
+      this.#content.copy(newContent, 0, 0, this.#size);
       this.#content = newContent;
     }
 
     // Write the data
     data.copy(this.#content, writePos);
 
+    // Update actual content size
+    if (neededSize > this.#size) {
+      this.#size = neededSize;
+    }
+
     // Update the entry's content and mtime
     if (this.#entry) {
-      this.#entry.content = this.#content;
+      this.#entry.content = this.#content.subarray(0, this.#size);
       this.#entry.mtime = DateNow();
     }
 
@@ -495,21 +506,27 @@ class MemoryFileHandle extends VirtualFileHandle {
 
     // In append mode, append to existing content
     if (this.flags === 'a' || this.flags === 'a+') {
-      const newContent = Buffer.alloc(this.#content.length + buffer.length);
-      this.#content.copy(newContent, 0);
-      buffer.copy(newContent, this.#content.length);
-      this.#content = newContent;
+      const neededSize = this.#size + buffer.length;
+      if (neededSize > this.#content.length) {
+        const newCapacity = MathMax(neededSize, this.#content.length * 2);
+        const newContent = Buffer.alloc(newCapacity);
+        this.#content.copy(newContent, 0, 0, this.#size);
+        this.#content = newContent;
+      }
+      buffer.copy(this.#content, this.#size);
+      this.#size = neededSize;
     } else {
       this.#content = Buffer.from(buffer);
+      this.#size = buffer.length;
     }
 
     // Update the entry's content and mtime
     if (this.#entry) {
-      this.#entry.content = this.#content;
+      this.#entry.content = this.#content.subarray(0, this.#size);
       this.#entry.mtime = DateNow();
     }
 
-    this.position = this.#content.length;
+    this.position = this.#size;
   }
 
   /**
@@ -530,7 +547,7 @@ class MemoryFileHandle extends VirtualFileHandle {
   statSync(options) {
     this.#checkClosed();
     if (this.#getStats) {
-      return this.#getStats(this.#content.length);
+      return this.#getStats(this.#size);
     }
     throw new ERR_INVALID_STATE('stats not available');
   }
@@ -552,17 +569,25 @@ class MemoryFileHandle extends VirtualFileHandle {
     this.#checkClosed();
     this.#checkWritable();
 
-    if (len < this.#content.length) {
-      this.#content = this.#content.subarray(0, len);
-    } else if (len > this.#content.length) {
-      const newContent = Buffer.alloc(len);
-      this.#content.copy(newContent, 0, 0, this.#content.length);
-      this.#content = newContent;
+    if (len < this.#size) {
+      // Zero out truncated region to avoid stale data
+      this.#content.fill(0, len, this.#size);
+      this.#size = len;
+    } else if (len > this.#size) {
+      if (len > this.#content.length) {
+        const newContent = Buffer.alloc(len);
+        this.#content.copy(newContent, 0, 0, this.#size);
+        this.#content = newContent;
+      } else {
+        // Buffer has enough capacity, just zero-fill the extension
+        this.#content.fill(0, this.#size, len);
+      }
+      this.#size = len;
     }
 
     // Update the entry's content and mtime
     if (this.#entry) {
-      this.#entry.content = this.#content;
+      this.#entry.content = this.#content.subarray(0, this.#size);
       this.#entry.mtime = DateNow();
     }
   }