Enforcing Sequential Consistency with Goroutines

In concurrent programming, ensuring that operations happen in a predictable order is crucial for correctness. This challenge focuses on achieving sequential consistency for a set of operations performed by multiple goroutines, preventing race conditions and guaranteeing that reads reflect writes in a defined order.

Problem Description

You are tasked with implementing a mechanism in Go that ensures a sequence of operations, initiated by multiple goroutines, is executed in a sequentially consistent manner. This means that if goroutine A performs an operation after goroutine B, any subsequent operation by goroutine B must be observed after the operation from A, from the perspective of all goroutines. Specifically, you need to synchronize a set of read and write operations on a shared variable such that the final state of the variable is deterministic and reflects the intended sequential order of these operations.

Key Requirements

1. Sequential Consistency: All goroutines must observe the operations in the same total order. If one goroutine performs an action A and another goroutine performs an action B, and A is causally related to B (e.g., A writes a value, and B reads that value), then all goroutines must see A happening before B.
2. Shared Variable Access: Multiple goroutines will perform read and write operations on a single integer variable.
3. Synchronization Mechanism: You must implement a synchronization mechanism (e.g., using mutexes, channels, or atomic operations) to enforce the sequential consistency.

Expected Behavior

When multiple goroutines concurrently attempt to read and write to a shared integer, the sequence of reads and writes should appear to have been executed atomically in some sequential order that respects the program's order within each individual processor. This means that a read operation should always return the value written by the most recent write operation that occurred before the read in the global sequential order.

Edge Cases

• Simultaneous Operations: Consider scenarios where multiple writes or a read and a write are initiated at roughly the same time.
• Empty Operations: What happens if no operations are performed? (Though less relevant for a practical challenge, conceptually important).
• Large Number of Goroutines/Operations: The solution should ideally scale well.

Examples

Example 1:

Input:
- Goroutine 1: Writes 5 to shared variable
- Goroutine 2: Reads shared variable
- Goroutine 3: Writes 10 to shared variable
- Goroutine 4: Reads shared variable

Possible Output:
Shared variable reads: [5, 10]
Explanation:
One possible sequential execution order is:
1. Goroutine 1 writes 5.
2. Goroutine 2 reads 5.
3. Goroutine 3 writes 10.
4. Goroutine 4 reads 10.
The key is that both reads observe values from writes that precede them in the *agreed-upon* global order. Another valid order might have Goroutine 3 write 10 before Goroutine 2 reads, but Goroutine 2 would still see the last write *before* its read in the sequential order.

Example 2:

Input:
- Goroutine 1: Writes 1
- Goroutine 2: Writes 2
- Goroutine 3: Reads
- Goroutine 4: Reads

Possible Output:
Shared variable reads: [1, 2]  or [2, 1] (depending on write order)
Explanation:
If the sequential order is G1 write 1, G2 write 2, G3 reads, G4 reads, then G3 and G4 will both read 2.
If the sequential order is G2 write 2, G1 write 1, G3 reads, G4 reads, then G3 and G4 will both read 1.
The crucial point is that both reads should observe the *same* value, which is the result of the last write in the established sequential order.

Example 3: Race Condition Prevention

Input:
- Goroutine 1: Writes 100
- Goroutine 2: Reads
- Goroutine 3: Writes 200
- Goroutine 4: Reads

Expected Output:
Shared variable reads: [100, 200] or [200, 100] (but not values in between, like 100 then 100 again if the second read happens before the second write)
Explanation:
Without proper synchronization, Goroutine 2 might read 0 (initial value), then Goroutine 1 writes 100, then Goroutine 4 reads 100, then Goroutine 3 writes 200. This is NOT sequentially consistent. A sequentially consistent system would ensure that if Goroutine 1's write is ordered before Goroutine 2's read, and Goroutine 3's write is ordered after Goroutine 2's read, then Goroutine 2 MUST see the result of Goroutine 1's write.

Constraints

• The shared variable is an int.
• There will be at least 2 goroutines and at least 2 operations (reads or writes).
• The total number of operations across all goroutines will not exceed 1000.
• The solution should avoid deadlocks.
• The implementation should be written entirely in Go.

Notes

• Consider the Go memory model and its guarantees.
• Think about how to establish a global order for operations that are not causally dependent.
• Channels can be used to pass messages and synchronize goroutines, effectively ordering operations.
• Mutexes (sync.Mutex) are fundamental for protecting shared data.
• Atomic operations (sync/atomic) can provide low-level primitives for certain types of synchronization.
• The goal is to simulate a system where all operations appear to execute one at a time, in some consistent global order.