feat(sdk): add NAS/OSS mount syntax sugar across Kotlin, Python, TypeScript, Go, and C#

Expose `sandbox.mount(NfsMountOptions | OssfsMountOptions)` and
`sandbox.umount(mountPoint)` on the sandbox facade in all five SDKs. The
helpers build the appropriate `mount -t nfs` / `ossfs` / `ossfs2` shell
command and run it via `commands.run`, so no server change is required. The
sandbox image must have the corresponding mount binary installed, or the
options.installation field can install it at mount time.

Cross-SDK behavior is aligned so callers can rely on the same semantics:

- Version selection: `OssfsMountOptions.version` defaults to ossfs 1.x when
  omitted, ossfs 2.x when set to "2.0".
- `bucketDirectory` is supported in both ossfs 1.x (`bucket:/dir`) and
  ossfs 2.x (`--oss_bucket_prefix=<dir>/`).
- STS `securityToken` appends to the ossfs 1.x password file and exports
  `OSS_SESSION_TOKEN` for ossfs 2.x.
- Failures raise a new `MountFailedException` / `MountFailedError` that
  extends the SDK's existing sandbox exception hierarchy and carries the
  failing `Execution` for diagnostics.

Security-hardened compared to the internal reference implementation:

1. The ossfs 1.x password file is created with mode 0600 atomically using
   `install -m 600 /dev/null /etc/ossfspass` before `printf %s <passwd>`,
   removing the world-readable window a naive `echo > file; chmod 600`
   would create.
2. The ossfs 1.x cleanup step is wrapped in `( ossfs ... ); __rc=$?;
   rm -f /etc/ossfspass; exit $__rc` so the password file is unlinked even
   when the ossfs binary itself exits non-zero.
3. The ossfs 2.x configuration file is written to
   `/tmp/opensandbox-ossfs-<uuid>.conf` with mode 600 rather than dropped
   in the current working directory.
4. All shell arguments are single-quoted with proper escaping.

While pinning the ossfs 2.x conf mode to 600 the existing SDK convention
was verified: `WriteEntry.mode` is serialized as a JSON number and parsed
server-side via `strconv.ParseUint(strconv.Itoa(mode), 8, 32)` (see
`components/execd/pkg/web/controller/utils.go`), so callers must pass the
decimal literal `600` rather than an octal literal such as `0o600` (which
would serialize to 384 and fail the octal parse). The Kotlin, TypeScript,
Go, and C# implementations all pass `600`; Python passes `mode=600` as
before.

The `createIsolatedOssSession` helper from the internal Java SDK (bwrap
seccomp workaround that pre-mounts OSSFS on the host side and bind-mounts
it into the isolated session) is intentionally out of scope of this
change and will be added separately if needed.

Test results (unit only; e2e requires real NAS/OSS backends):

- Kotlin: `./gradlew :sandbox:test` -> 216 pass
- Python: `uv run pytest tests/` -> 302 pass
- TypeScript: `pnpm run test` -> 70 pass
- Go: `go test ./...` -> all pass
- C#: not verified locally (no dotnet SDK available on the workstation);
  code is written to match the existing patterns in the C# SDK and passes
  static review, but `dotnet test` still needs to run in CI.

Author: Pangjiping

sdks/sandbox/csharp/README.md

@@ -226,6 +226,40 @@ foreach (var s in list.Items)
 }
 ```
 
+### NAS / OSS Mounts
+
+`sandbox.MountAsync(...)` runs `mount -t nfs`, `ossfs`, or `ossfs2` inside a running sandbox via `Commands.RunAsync`, so the sandbox image must have the corresponding binary installed (or use `Installation` to install it on demand). The password / configuration files are created with mode 600 in one step and are cleaned up even when the mount command itself fails.
+
+```csharp
+using OpenSandbox;
+using OpenSandbox.Models;
+
+// Mount a NAS export
+await sandbox.MountAsync(new NfsMountOptions
+{
+    Endpoint = "nas-server.example.com",
+    NasPath = "/share",
+    MountPoint = "/mnt/nas",
+    Installation = "apt-get install -y nfs-common", // optional
+});
+
+// Mount an OSS bucket via ossfs 2.x
+await sandbox.MountAsync(new OssfsMountOptions
+{
+    Endpoint = "https://oss-cn-hangzhou.aliyuncs.com",
+    Bucket = "my-bucket",
+    BucketDirectory = "subdir",             // optional; maps to --oss_bucket_prefix
+    MountPoint = "/mnt/oss",
+    AccessKeyId = Environment.GetEnvironmentVariable("OSS_AK")!,
+    AccessKeySecret = Environment.GetEnvironmentVariable("OSS_SK")!,
+    Version = OssfsVersion.Ossfs20,
+});
+
+await sandbox.UmountAsync("/mnt/nas");
+```
+
+Failures throw `MountFailedException` (`OpenSandbox.Core`), which exposes the failing `Execution` on `ex.Execution` so callers can inspect exit code and stderr.
+
 ## Configuration
 
 ### 1. Connection Configuration

sdks/sandbox/csharp/src/OpenSandbox/Core/Exceptions.cs

@@ -43,6 +43,11 @@ public static class SandboxErrorCodes
     /// Unexpected response from the server.
     /// </summary>
     public const string UnexpectedResponse = "UNEXPECTED_RESPONSE";
+
+    /// <summary>
+    /// A NAS or OSS mount syntax-sugar call failed inside the sandbox.
+    /// </summary>
+    public const string MountFailed = "MOUNT_FAILED";
 }
 
 /// <summary>
@@ -231,3 +236,34 @@ public InvalidArgumentException(string? message = null, Exception? innerExceptio
     {
     }
 }
+
+/// <summary>
+/// Exception thrown when a NAS or OSS mount syntax-sugar call fails inside the sandbox.
+/// </summary>
+/// <remarks>
+/// The <see cref="Execution"/> property exposes the failing execution handle
+/// returned by the underlying <c>Commands.RunAsync</c> call so callers can inspect
+/// the exit code, stdout, and stderr for diagnostics.
+/// </remarks>
+public class MountFailedException : SandboxException
+{
+    /// <summary>
+    /// Gets the failing execution result attached to this exception, if any.
+    /// </summary>
+    public OpenSandbox.Models.Execution? Execution { get; }
+
+    /// <summary>
+    /// Initializes a new instance of the <see cref="MountFailedException"/> class.
+    /// </summary>
+    /// <param name="message">The error message.</param>
+    /// <param name="execution">The failing execution result.</param>
+    /// <param name="innerException">The inner exception.</param>
+    public MountFailedException(
+        string? message = null,
+        OpenSandbox.Models.Execution? execution = null,
+        Exception? innerException = null)
+        : base(message, innerException, new SandboxError(SandboxErrorCodes.MountFailed, message))
+    {
+        Execution = execution;
+    }
+}

sdks/sandbox/csharp/src/OpenSandbox/Internal/MountShell.cs

@@ -0,0 +1,287 @@
+// Copyright 2026 Alibaba Group Holding Ltd.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+using System.Globalization;
+using System.Text;
+using OpenSandbox.Core;
+using OpenSandbox.Models;
+
+namespace OpenSandbox.Internal;
+
+/// <summary>
+/// Internal builders that translate mount option objects into shell commands.
+/// Exposed as <c>internal</c> so the unit tests in <c>OpenSandbox.Tests</c> can
+/// call them directly via <c>InternalsVisibleTo</c>.
+/// </summary>
+internal static class MountShell
+{
+    /// <summary>
+    /// Path inside the sandbox where ossfs 1.x reads its password file.
+    /// </summary>
+    public const string Ossfs1PasswdPath = "/etc/ossfspass";
+
+    public sealed record Ossfs2Plan(string ConfPath, string ConfContent, string Command);
+
+    public static void ValidateNfs(NfsMountOptions options)
+    {
+        if (options is null)
+        {
+            throw new ArgumentNullException(nameof(options));
+        }
+        if (string.IsNullOrWhiteSpace(options.Endpoint))
+        {
+            throw new InvalidArgumentException("endpoint must not be blank");
+        }
+        if (string.IsNullOrWhiteSpace(options.MountPoint))
+        {
+            throw new InvalidArgumentException("mountPoint must not be blank");
+        }
+        if (string.IsNullOrWhiteSpace(options.NasPath))
+        {
+            throw new InvalidArgumentException("nasPath must not be blank");
+        }
+    }
+
+    public static void ValidateOssfs(OssfsMountOptions options)
+    {
+        if (options is null)
+        {
+            throw new ArgumentNullException(nameof(options));
+        }
+        if (string.IsNullOrWhiteSpace(options.Endpoint))
+        {
+            throw new InvalidArgumentException("endpoint must not be blank");
+        }
+        if (string.IsNullOrWhiteSpace(options.Bucket))
+        {
+            throw new InvalidArgumentException("bucket must not be blank");
+        }
+        if (string.IsNullOrWhiteSpace(options.MountPoint))
+        {
+            throw new InvalidArgumentException("mountPoint must not be blank");
+        }
+        if (string.IsNullOrWhiteSpace(options.AccessKeyId))
+        {
+            throw new InvalidArgumentException("accessKeyId must not be blank");
+        }
+        if (string.IsNullOrWhiteSpace(options.AccessKeySecret))
+        {
+            throw new InvalidArgumentException("accessKeySecret must not be blank");
+        }
+    }
+
+    public static string BuildNfsCommand(NfsMountOptions options)
+    {
+        var optString = string.IsNullOrWhiteSpace(options.Options)
+            ? NfsMountOptions.DefaultNfsOptions
+            : options.Options!;
+        var source = options.Endpoint + ":" + options.NasPath;
+        var core = string.Format(
+            CultureInfo.InvariantCulture,
+            "mkdir -p {0} && mount -t nfs -o {1} {2} {3}",
+            ShQuote(options.MountPoint),
+            ShQuote(optString),
+            ShQuote(source),
+            ShQuote(options.MountPoint));
+        return PrependInstallation(options.Installation, core);
+    }
+
+    public static string BuildOssfs1Command(OssfsMountOptions options)
+    {
+        var useSts = !string.IsNullOrWhiteSpace(options.SecurityToken);
+        // ossfs 1.x password file format:
+        //   AK/SK mode : bucket:accessKeyId:accessKeySecret
+        //   STS mode   : bucket:accessKeyId:accessKeySecret:securityToken
+        var passwd = new StringBuilder();
+        passwd.Append(options.Bucket).Append(':')
+              .Append(options.AccessKeyId).Append(':')
+              .Append(options.AccessKeySecret);
+        if (useSts)
+        {
+            passwd.Append(':').Append(options.SecurityToken);
+        }
+
+        var bucketArg = string.IsNullOrWhiteSpace(options.BucketDirectory)
+            ? options.Bucket
+            : $"{options.Bucket}:/{options.BucketDirectory}";
+
+        var optionFlags = new StringBuilder();
+        if (options.Options is { Count: > 0 } opts)
+        {
+            foreach (var opt in opts)
+            {
+                optionFlags.Append(" -o").Append(ShQuote(opt));
+            }
+        }
+
+        // Create the password file with mode 0600 in one atomic step (avoids the
+        // world-readable window that a naive `echo > ; chmod 600` would create).
+        // Always clean up the password file even if ossfs fails, by preserving
+        // the subshell exit code via __rc.
+        var core = string.Format(
+            CultureInfo.InvariantCulture,
+            "ossfs --version && " +
+            "install -m 600 /dev/null {0} && " +
+            "printf %s {1} > {0} && " +
+            "mkdir -p {2} && " +
+            "( ossfs {3} {2} -ourl={4} -opasswd_file={0}{5} ); " +
+            "__rc=$?; rm -f {0}; exit $__rc",
+            Ossfs1PasswdPath,
+            ShQuote(passwd.ToString()),
+            ShQuote(options.MountPoint),
+            ShQuote(bucketArg),
+            ShQuote(options.Endpoint),
+            optionFlags.ToString());
+        return PrependInstallation(options.Installation, core);
+    }
+
+    public static Ossfs2Plan BuildOssfs2Plan(OssfsMountOptions options)
+    {
+        var conf = new StringBuilder();
+        conf.Append("--oss_endpoint=").Append(options.Endpoint).Append('\n');
+        conf.Append("--oss_bucket=").Append(options.Bucket).Append('\n');
+        if (!string.IsNullOrWhiteSpace(options.BucketDirectory))
+        {
+            // ossfs2 mounts a bucket root; a subdirectory is expressed as a
+            // prefix (trailing slash makes it a directory boundary).
+            var prefix = options.BucketDirectory!.TrimEnd('/') + "/";
+            conf.Append("--oss_bucket_prefix=").Append(prefix).Append('\n');
+        }
+        if (options.Options is { Count: > 0 } opts)
+        {
+            foreach (var opt in opts)
+            {
+                conf.Append("--").Append(opt).Append('\n');
+            }
+        }
+
+        var confPath = "/tmp/opensandbox-ossfs-" + Guid.NewGuid().ToString("N") + ".conf";
+
+        // ossfs 2.x reads credentials from environment variables:
+        //   AK/SK mode : OSS_ACCESS_KEY_ID + OSS_ACCESS_KEY_SECRET
+        //   STS mode   : additionally OSS_SESSION_TOKEN
+        var stsExport = string.IsNullOrWhiteSpace(options.SecurityToken)
+            ? string.Empty
+            : $" && export OSS_SESSION_TOKEN={ShQuote(options.SecurityToken!)}";
+
+        var core = string.Format(
+            CultureInfo.InvariantCulture,
+            "ossfs2 --version && " +
+            "mkdir -p {0} && " +
+            "export OSS_ACCESS_KEY_ID={1} && " +
+            "export OSS_ACCESS_KEY_SECRET={2}{3}" +
+            " && ossfs2 mount {0} -c {4}",
+            ShQuote(options.MountPoint),
+            ShQuote(options.AccessKeyId),
+            ShQuote(options.AccessKeySecret),
+            stsExport,
+            ShQuote(confPath));
+
+        return new Ossfs2Plan(
+            ConfPath: confPath,
+            ConfContent: conf.ToString(),
+            Command: PrependInstallation(options.Installation, core));
+    }
+
+    public static WriteEntry BuildOssfs2ConfEntry(Ossfs2Plan plan)
+    {
+        if (plan is null)
+        {
+            throw new ArgumentNullException(nameof(plan));
+        }
+        return new WriteEntry
+        {
+            Path = plan.ConfPath,
+            Data = plan.ConfContent,
+            // Mode is serialized as a JSON number and parsed by execd as an
+            // octal string (see components/execd/pkg/web/controller/utils.go);
+            // pass the literal decimal 600 rather than the C# hex/binary form
+            // for octal 0o600.
+            Mode = 600,
+        };
+    }
+
+    public static string BuildUmountCommand(string mountPoint)
+    {
+        if (string.IsNullOrWhiteSpace(mountPoint))
+        {
+            throw new InvalidArgumentException("mountPoint must not be blank");
+        }
+        return "umount " + ShQuote(mountPoint);
+    }
+
+    public static OssfsVersion SelectOssfsVersion(OssfsMountOptions options)
+    {
+        return options.Version ?? OssfsVersion.Ossfs10;
+    }
+
+    public static void EnsureSuccess(Execution? execution, string failurePrefix)
+    {
+        if (execution is null)
+        {
+            throw new MountFailedException(failurePrefix + ": nil execution result");
+        }
+        var error = execution.Error;
+        var exitCode = execution.ExitCode;
+        var failed = error != null || (exitCode.HasValue && exitCode.Value != 0);
+        if (!failed)
+        {
+            return;
+        }
+        var parts = new List<string>(2);
+        if (error != null)
+        {
+            parts.Add($"[{error.Name}] {error.Value}");
+        }
+        if (execution.Logs.Stderr.Count > 0)
+        {
+            var sb = new StringBuilder();
+            for (var i = 0; i < execution.Logs.Stderr.Count; i++)
+            {
+                if (i > 0)
+                {
+                    sb.Append('\n');
+                }
+                sb.Append(execution.Logs.Stderr[i].Text);
+            }
+            parts.Add("stderr=" + sb);
+        }
+        var message = parts.Count == 0
+            ? failurePrefix
+            : failurePrefix + ": " + string.Join(" | ", parts);
+        throw new MountFailedException(message, execution);
+    }
+
+    private static string PrependInstallation(string? installation, string core)
+    {
+        return string.IsNullOrWhiteSpace(installation) ? core : installation + " && " + core;
+    }
+
+    /// <summary>
+    /// Quotes <paramref name="value"/> for POSIX shell single-quoted context.
+    /// Embedded <c>'</c> is escaped as <c>'\''</c>. The result is always safe to
+    /// embed as one argument to a <c>sh -c</c> string.
+    /// </summary>
+    public static string ShQuote(string value)
+    {
+        if (value is null)
+        {
+            throw new ArgumentNullException(nameof(value));
+        }
+        // string.Replace(string, string) is available on all target frameworks
+        // (including netstandard2.0). Ordinal comparison is the default for
+        // this overload.
+        return "'" + value.Replace("'", "'\\''") + "'";
+    }
+}