deploy-to-aca.ps1: StateStore connection string truncated to first character on macOS/Linux

[sebastianstephan]

When running deploy-to-aca.ps1 on macOS, the StateStore connection string is truncated to its first character, causing the web app start to fail with:

[2026-02-26T14:25:13.207Z] Error: System.InvalidOperationException: Failed to initialize DocumentDBPersistence. Details: MongoDB.Driver.MongoConfigurationException: The connection string '"m"' is not valid.
   at MongoDB.Driver.Core.Configuration.ConnectionString.Parse()
   at MongoDB.Driver.Core.Configuration.ConnectionString..ctor(String connectionString, Boolean isInternalRepresentation, IDnsResolver dnsResolver)
   at MongoDB.Driver.Core.Configuration.ConnectionString..ctor(String connectionString)
   at MongoDB.Driver.MongoUrlBuilder.Parse(String url)
   at MongoDB.Driver.MongoUrlBuilder..ctor(String url)
   at MongoDB.Driver.MongoUrl..ctor(String url)
   at MongoDB.Driver.MongoClientSettings.FromConnectionString(String connectionString)
   at MongoDB.Driver.MongoClient..ctor(String connectionString)
   at OnlineMongoMigrationProcessor.Persistence.DocumentDBPersistence.Initialize(String connectionStringOrPath, String appId) in /src/OnlineMongoMigrationProcessor/Persistence/DocumentDBPersistence.cs:line 110
 ---> MongoDB.Driver.MongoConfigurationException: The connection string '"m"' is not valid.
   at MongoDB.Driver.Core.Configuration.ConnectionString.Parse()
   at MongoDB.Driver.Core.Configuration.ConnectionString..ctor(String connectionString, Boolean isInternalRepresentation, IDnsResolver dnsResolver)
   at MongoDB.Driver.Core.Configuration.ConnectionString..ctor(String connectionString)
   at MongoDB.Driver.MongoUrlBuilder.Parse(String url)
   at MongoDB.Driver.MongoUrlBuilder..ctor(String url)

Root Cause:

The issue stems from two bugs in the connection string handling:

Wrong marshalling method for BSTR on non-Windows platforms

SecureStringToBSTR returns a pointer to a BSTR, which is always UTF-16 encoded — every ASCII character occupies 2 bytes (e.g., m → 0x6D 0x00). PtrToStringAuto selects encoding based on the platform: on Windows it uses UTF-16 (which happens to work), but on macOS/Linux (.NET Core) it uses UTF-8. When interpreting the UTF-16 bytes as UTF-8, it reads 0x6D (m) followed by 0x00 (null terminator) and stops immediately, yielding only the first character.

Additionally, the BSTR pointer was never freed, causing a memory leak.

Escaped quotes around the parameter value

The connection string was wrapped in escaped double quotes before being passed as a Bicep parameter, so the value m became "m" (with literal quote characters) — which is what the MongoDB driver reports as invalid.

Fix:

Replace PtrToStringAuto with PtrToStringBSTR, which correctly reads the BSTR length prefix and decodes the full UTF-16 string regardless of platform.
Free the BSTR pointer with ZeroFreeBSTR in a finally block to prevent memory leaks.
Remove the escaped quotes from the Bicep parameter value.
Impact: Deployment via deploy-to-aca.ps1 fails on all non-Windows platforms. Windows users are unaffected because PtrToStringAuto coincidentally uses UTF-16 there.

[sebastianstephan]

solved in PR #211